Chapter 1

The Development of the Imperial Japanese Navy’s Fighter Force

Before describing the design and combat of the Mitsubishi A6M carrier-borne fighter aircraft of World War II one needs to know just how this aircraft came to be what it was like when she first appeared in the eastern skies. The modern world is very different to that of 1937 and so in order to understand this aircraft’s pedigree the origins and history that made her so unique have to be understood. For that a brief background as to why Japan was in China at all, and why a naval aircraft was to dominate land and sea warfare, needs to be outlined.

The origins of the Imperial Japanese Navy’s air arm can be established back to 1912 with the foundation of the Commission on Naval Aeronautical Research (Kaigun Kōkūjutsu Kenkyūkai) set up under the Technical Development Department. On 19 December Captains Yoshitoshi Tokugawa and Kumazo Hino, conducted the first power flights in a Henri Farman biplane and Grade monoplane respectively. Like its long-time mentor, the Royal Navy, initially much early work was concentrated on airships, but with developments during World War I, emphasis shifted to winged and powered flying machines. Aircraft were purchased from both France and the United States and pilot training commenced also from 1912 onward. From 1914, allied as it was to Great Britain, Japan assisted in the conquest of the German treaty port and base of Tsingtao in November 1914, replacing that power there for eight more years rather than handing it back to China. The Japanese used airships in the siege as well as the seaplane-carrier Wakamiya and co-operation between the two navies continued, a flotilla of Japanese destroyers being based at Malta during the war. However, the Japanese Army was equally as strongly pro-German and envisaged the mutually destructive European carnage as an enormous opportunity to exploit the weakness of China while attentions were focused elsewhere. The Japanese duly presented what were known as the ‘Twenty-One Demands’ to the Chinese Government, which, if implemented, would have given Japan unprecedented control of Chinese internal affairs and commerce and much territory. The Americans also entered the war in 1917 and thus technically became an ally of Japan and, like the British, their initial objections to this take-over were watered-down, with the American Under-Secretary of State, Robert Lansing, stating to Viscount Ishii Kikujirō in a note of 2 November 1917 that America now recognized that, ‘… territorial propinquity creates special relations between nations and consequently Japan has special interests in China’. This became known as the Lansing-Ishii agreement, which virtually gave Japan carte blanche to proceed with her ambitions, while secretly agreeing to extend both nations’ influences in China at the expenses of all other powers.

After the Great War ended with Germany’s unexpected collapse, Japan benefitted from her involvement on the Allied side by taking over the Mandates of Germany’s former Pacific colonies in the Caroline, Mariana and Marshall islands. But these acquisitions did not sate her expansionist plans in China and Manchuria. Meanwhile, Japan still remained allied to Great Britain and Royal Navy influence dominated the expansion of her naval aircraft development. From April 1916 Naval Air Units (hikōtai) were established by the Naval Affair Bureau, at Yokosuka and then Kasumigaura. The British experiments with the carriers Furious and Argus in the latter part of the Great War naturally aroused the interest of the Japanese as did the laying down of the first fully designed aircraft carrier, the Hermes. The Americans at this stage were yet to convert the collier Jupiter into the carrier Langley and lagged far behind the British. The Japanese response was to build the Hōshō, and she, due to the malaise that gripped British defence policy post-war, beat the Hermes into commission to become the first operational aircraft carrier built as such. The dead hand of RAF control soon stifled the initial British lead and both Japan and America quickly overtook and then left the Royal Navy far behind in naval aircraft development, but in the early 1920s it was still to her ally Britain that Japan looked.

Japanese naval aviators flew from the Furious in 1920 and Britain sent an ‘unofficial’ Naval Air Mission under the Colonel The Master of Sempill, with thirty former RNAS and RAF pilots to Japan between September 1921 and 1922 to train a cadre of naval flyers in the latest techniques. When Hōshō commissioned it was again a British flyer, William Jordan, a former RNAS and then test pilot for the Sopwith Company, who, in February 1923, made the first deck landing flying a home-built Mitsubishi aircraft. It was not until 16 March that a Japanese pilot, Lieutenant Shunichi Kira, performed the same feat. Under Herbert Smith former Sopwith flyers such as Jordan, along with Jutland hero Frederick J. Rutland (later to become a spy for Japan), introduced the latest techniques and machines such as the Mitsubishi 1MF3 Type 10 Kōsen (carrier fighter). Herbert Smith had been the Chief Aircraft Designer for the British Sopwith Aviation Company. Among the most famous of his many successful warplane designs during World War I had been the Sopwith Pup and Sopwith Camel. When Sopwith’s disappeared in the harsh climate of British post-war austerity, Smith was invited to Japan by the Mitsubishi Engine Manufacturing Company, at Nagoya, along with other former Sopwith engineers whose expertise was more prized in Japan than in the UK. This team helped Mitsubishi establish their Aircraft Division and Smith oversaw three major Japanese aircraft designs before retiring and returning home in 1924. This was a single-engined single-seat biplane powered by a 300hp Mitsubishi 8-cylinder water-cooled engine and armed with two forward-firing fixed Type 97 7.7mm light machine guns (LMG), and had a maximum speed of 132mph. Another early Japanese model carrier fighter was the Nakajima A1N1/A1N2 Type 3 fighter, based firmly on the one-off speculative British Gloster Gambet, itself a carrier version of the Gloster Gamecock, and which had first flown on 12 December 1927 and was sent to Japan the following year. The A1N was licence-built, and of similar wood and fabric design but with a 420hp Nakajima Jupiter VI 9-cylinder air-cooled radial engine as power plant, which could attain a top speed of 148mph and also carry two 66lb bombs for dive-bombing. Some 150 were built, the first entering service in 1939. A further advance was the Nakajima A2N1 Type 90 carrier fighter. She was also a single-seater and she was a composite design with an all-metal fuselage construction with metal fabric-covered wings that could attain a maximum speed of 182mph.

British aircraft were the inspiration for early Japanese designers, with the Yokosuka Naval Air Arsenal the first manufacturer of such types quickly joined by Aichi, Mitsubishi and Nakajima. They soon began producing aircraft that were at least the equals of western types. Japan was full of confidence and looked forward to continued expansion of her dominions and power. She had already annexed Korea in 1910 and was looking for further additions to her overseas territories to maintain her expanding population and provide sources for coal and iron to fuel her growth. Two revolutions, one against the Manchu Dynasty in China in 1911 and one for its restoration in 1913, both had the hidden hand of the Japanese behind them who sought to benefit from the destabilization of that state. Now, in 1919, the ‘Open Door’ policy of access to China was increasingly unacceptable to Japan who sought predominance and the exclusion of all other powers.

Then, in 1921, an unexpected intervention happened on the world stage that eclipsed even the Versailles Treaty; the Americans suddenly called a conference to limit naval shipbuilding and produced a set of ratio figures, in a total arbitrary manner, totally without any consultation whatsoever, and designed to ensure their own domination of the seaways. This followed the vast programme of naval construction initiated by President Wilson designed to elevate the US to be the principal power, based on their own self-image (one not shared by the rest of the world) of ‘The leader of the democratic impulse’. This assembly became the generally known as the Washington Naval Conference.

The United States continued to castigate and hector the other delegates during the resulting conference to accept the United States’ parity with Great Britain and also assigned tonnage limits on the other world powers.¹ Negotiations were long and acrimonious, especially on the part of Japan who felt that her natural progression to join the leading naval powers and her much-cherished ‘Eight-Eight’ naval shipbuilding programme were needlessly sacrificed.² In contrast, myopic British politicians, headed by Tory politician Arthur J. Balfour, urged on by an all-powerful Treasury Department, meekly caved in to the American demands and abandoned Britain’s three hundred-year-old dominance of the sea with hardly a murmur; indeed they wanted to go further so that Beatty, the First Sea Lord, wrote in despair that if things continued ‘… there would not be a Navy at all’.³ The Japanese were less accommodating to the diktat from President Warren G Harding in the White House and his team backed by demands from Senators from such states as Iowa and Nebraska thousands of miles from any ocean. That the Japanese signed at all was remarkable, as it reduced their total tonnage to just two-thirds of the new American and British limits. While British politicians meekly accepted this ratio, the reception received by the returning Japanese delegates, under Katō Tomosaburō, was dire. Most admirals, the so-called ‘Big-Navy’ group, and in particular Vice-Admiral Katō Kanji, who had also been a delegate member, were bitterly opposed to the deal, and made no secret of it. Another by-product that favoured the Americans and nobody else was that the Anglo-Japanese alliance, which was due to be renewed that same year, was allowed to lapse. Great Britain therefore lost her naval dominance and her principal naval ally both at the same time – for this outcome Balfour received an earldom! In Japan one effect that the resulting treaty had was that it concentrated the minds of the Japanese Naval Staff on how to offset this deliberate restriction of their ambition, and in the ensuing years they increasingly tended to see the development of their naval air arm as a way of reducing the balance of American dominance. Two of the abandoned capital ships, the battleship Kaga and the battle-cruiser Akagi, were converted on the stocks to become aircraft carriers.⁴

This switch was further reinforced by the subsequent London Naval Treaty of 1930 in which restriction on tonnages and armament size for cruisers was extended by a limitation on numbers, in roughly the same ratio as for battleships and carriers. This was another blow to Japan, which had sought to build outstanding cruisers to make up for the enforced deficiency in her battle line. Now that expansion was also thwarted the air option increasingly became the attractive option for parity in some fields. In 1924 a brand-new Aviation Research centre was constructed at Kasumigaura to replace one demolished in the Tokyo earthquake, to centralize development, testing and research, although existing facilities at Hirō and a new engine testing centre was opened at Yokosuka. Further development took place here with the building of the new Naval Air Arsenal completed six years later. Once ready this new facility became the focus of the IJN’s naval air expansion, allowing both the existing Technical Section and the Aviation Department of the Navy to share design, development, manufacture and flight-testing, via the Hikō Jikkkenbu right through to training and tactical application by the Yokosuka Kōkūtai, not only of all non-private aircraft types but of the hardware associated with military aircraft as well. Such an integrated facility as the Kaigun kōkūtai (re-named as the Naval Air Technical Arsenal – Kaigun Kōkū Gijutsushō – in 1939) was an enormous asset to the Navy and enabled combat experience to be readily assimilated and built into new designs.⁵ Compare this seamless progression and total control of all aspects of naval air development with the convoluted and time-wasting process by which the Admiralty had to go cap-in-hand to the Air Ministry with their requests, which the latter had barely concealed contempt for (and a overwhelming aversion to consider any military option other than laying Berlin in ruins), and with a hostile Treasury and pacifist government opposing both. It is little wonder that the Royal Navy’s air arm withered while that of the IJN waxed stronger and more powerful each year.

Through this new centralized command the Navy, which contained many air-minded admirals (some of whom, like Vice-Admiral Inoue Shigeyoshi, wanted the entire navy to be solely a land-based aerial armada), was able to channel all Japan’s limited resources to achieving two common goals; to cease reliance on foreign designs and to establish air superiority for the fleet. This superiority was not confined to carrier aircraft either; modern long-range bombers, capable of both high-altitude attacks and aerial torpedo attacks, were developed as a fully integrated force rather than as separate entities. This forward-looking policy was named Project Aviation Technology Independence⁶ and was carried through with vim and vigour. Private companies were encouraged to compete and from 1932 under the new Prototypes System, under which maximum effort was attained by controlled competition, manufacturers were made to bid in pairs on new designs, outline requirements of which were laid down by the Navy. The best of each competitor’s designs were then adopted, but there were no losers for the firm whose design was rejected was permitted to construct their rival’s machine, or to provide engines for it, thus keeping both fully engaged and able to maximize output and resources.

Hand-in-hand with these innovations that spurred the indigenous aircraft industry forward and enabled the Japanese to cease to rely on foreign technologies to a large extent, was an expansion of pilot and aircrew training in order to meet a manning requirement for almost one thousand aircraft by 1938.⁷

The development of the bombing arm was consistent with the traditional Japanese approach of attack over defence. This led to the adoption of dive-bombing and torpedo-bombing by the carrier embarked aircraft as well as the long-range bomber, and this tendency somewhat overshadowed the fighter force, which was seen as purely defensive. This was not just a Japanese trait however, the Italian General Giulio Douhet espoused the same philosophy in Europe,⁸ while Lord Hugh Trenchard in Britain wedded the whole of the RAF’s doctrine to the heavy bomber ethos with Air Marshal Arthur Harris carrying out this theory in bloody detail during the war. American thinking was very much the same with the gimmicky exhibitionism of Billy Mitchell grabbing headlines in a series of very one-sided tests against unmanned, stationary, obsolete warship targets and the predominance of the United States Army Air Corps ‘Bomber Mafia’ under Harold L. George and John F. Curry at Langley Field. All were to be proved wrong in the years ahead but the Japanese came earlier than most to the realization that the popular mantra ‘The bomber will always get through’, as propagated by British Prime Minister Stanley Baldwin in 1932, was a typically simplistic political fallacy.

The reason the Japanese were far ahead of the pack in this realization was because in the years between 1918 and 1937 she had continued her expansionist policy in the Far East and had become embroiled in a series of interventions that were, in all but name, wars.

It is typical that, in an oblique apology at the beginning of their book, authors Masatake Okumiya and Jirō Horikoshi stated that Japan had been, on occasions, ‘… forced into armed conflict with neighbouring countries’. They add that, ‘… such action may be justified …’, but little or no justification was apparent in the first of such adventures, which they totally ignore, the Liutiaohu Incident, which was nothing less than a plot to take over, through force, the whole of Manchuria. On 18 September 1931, a group of disaffected and militant Kwantung Army officers originated a plan whereby one Lieutenant Kawamoto Suemori planted explosives that blew up the Japanese owned South Manchuria Railway line between Changchun and Lüshun; using this as an excuse, they marched in and took over Mukden, the capital. Here, they put in place a puppet ruler Emperor Kang-de (formerly Puyi of the Quing Dynasty), and created a new subservient state, which they named Manchukuo. At the Liaodong Peninsula in the south (Dairen and the former Port Arthur or Ryojun) Japan ruled directly as the Kwantung Leased Territory and based their Kwantung Army (Kantōgun) there. Thus Japan had gained a firm military foothold on the mainland with Kange-de as a nominal figurehead for a nation that, in reality, the Japanese Army really administered.

In 1932 Japanese naval forces at Shanghai were involved in another incident when, with the beating and murder of Japanese Buddhist priests and anti-Japanese demonstrations as an excuse, an armed force was landed from warships at the port. Unfortunately for them they encountered the Chinese 19th Route Army and had to be hastily reinforced by regular army units sent from Japan. There was much fighting before the matter was settled by negotiation thanks to the British Admiral on the spot, Sir Howard Kelly, acting alone with little support from either his Government at home or the senior Admiral on the spot (an American) who refused to get involved. Meanwhile, the Japanese had annexed Jehol Province into Manchukuo in 1933 and in 1935 another puppet state, the East Hebei Autonomous Council, was set up absorbing the provinces of Chahar and Hebei. This, in effect, almost isolated the Chinese capital of Peking (now Beijing) other than from the south.

Five years later things turned more serious and on 7 July 1937 the so-called Marco Polo (Lugouqiao) Bridge incident occurred. Here the Peking (Beijing) to Tianjin railway ran through the town of Wanping, which the Japanese had long set their sights on. This friction point ruptured on the night of 6/7 July when the Japanese Kwantung Army conducted night manoeuvres without notifying their Chinese opposite numbers, who opened fire on the Japanese. The misunderstandings multiplied and fighting broke out on a large scale and the fighting soon spread to Shanghai once more. This began a bitter and costly war between Japan and China that was to last for eight long years. From the Navy’s viewpoint the isolation of its garrison in Shanghai led to support from both carrier-based aircraft (the carriers Hōshō, Kaga and Ryūjō were on station) and raids by land-based Mitsubishi G3M2 Navy Type 96 Model 22 bombers (rikkō) based on Taihoku, Formosa, and from Ōmua, Kyüshü, the first ‘transoceanic’ bombing missions directed against the cities of Hangchow, Nanking and Kuang-te. However, in contrast to the Douhet theorists’ claims, these bombing raids from both types of Japanese aircraft soon began to suffer very heavy losses when intercepted by fighter aircraft of the Republic of China Air Force. The Chinese airmen were heavily reliant on foreign-built, designed and supplied military aircraft, of which they had around 250 machines in 1937. American-built Boeing P-26C (Peashooter) and Curtiss Hawk II and III fighters, some of which were built at Hangzhou, predominated along with British Gloster Gladiators and, later, Russian Polikarpov I-15 and I-16B ‘Donkey’ fighters. Among a motley collection of other types were the Italian Fiat CR.30, American Republic P-43 Lancers and a lone Vultee P-66 Vanguard. Both the Soviet Union and America provided ‘volunteer’ flyers to assist in manning them.

In response to their losses the Japanese hastily replaced their Nakajima Type 90 and 95 biplane fighters, which had proved ineffective, with the still experimental Mitsubishi A5M4 Type 96 in the Second Combined Kōkū Sentai, later to be based at Kunda, Shanghai. This little fighter was an all-metal, open cockpit, single-seater monoplane with a fixed undercarriage and powered by either a Mitsubishi 730hp or a Nakajima Kotobuki 41 750hp radial engine, which gave her a top speed of 270mph and a range of 460 miles. She was armed with two 7.7mm Type 89 machine-guns. This feisty little aircraft had been designed by a young Mitsubishi chief designer, a graduate from Tokyo University, named Jirō Hortikoshi. His first design, Prototype 7, had not been a success but he was trusted with the new machine and the Prototype 9 had been completed in January 1935 and was exhaustively flight-tested at the Kagamigahara field and later by the nine pilots of the ‘Genda Circus’ at Yokosuka. The then Lieutenant Minoru Genda had assembled a group of highly skilled and adept fighter pilots at this period. They put each new model through its paces. At that time the Navy’s priority was for high-manoeuvrability, required for the Japanese method of dog-fighting, which itself was terminology based on the Great War’s type of aerial combat between opposing fighter aircraft. A modern description is air combat manoeuvring (ACM), or ‘turning combat’ but the old term is still widely understood and used. In Europe and America in the 1930s this concept was largely abandoned in theory, but World War II and afterwards saw the tactic, if not the name, widely used until the 1990s. The Japanese interpretation, tested and perfected by Petty Officer Mochizuki Isamu of the Yokosuka Kōkūtai as early as 1934, was the hineri-komi (‘in-turning’ or in Allied parlance, ‘falling-leaf ’), manoeuvre, (often reported by Allied survivors who did not understand it as ‘acrobatics’) by which targeted aircraft that went into an avoiding loop could be brought back into the attacker’s gun-sight by carrying out an instant cork-screwing ‘inner-loop’ before they knew what was happening. One A6M pilot described it this way: ‘It was executed near the top of a loop by applying cross control (aileron: right, rudder: left) for a short moment. But I have never used it on actual combat, because the plane would be slowed down at the top of a loop, making the plane an easy prey for other enemy fighters.’⁹ Attaining such an individual skill set by a combination of intensive training and an aircraft that was agile to the nth degree, perfectly suited to the Japanese pseudo-Samurai warrior duelling ethic, was based on a flexible, loose three-plane formation (shōtai), something that, hitherto, had best been obtained from a biplane configuration and so Hortikoshi’s monoplane prototype was something of a watershed. Having already exceeded the Navy’s requirement for a high top speed, the Genda team tested the Prototype 9 against a range of foreign designs, including the British Hawker Nimrod, German Heinkel He.112, American Severesky 2PA and French Dewoitine D510, as well as the existing Nakajima Type 95. The new aircraft, a product of the Mitsubishi Jūkōgyō Kakbushiki-Kaisha (Mitsubishi Heavy Industries Company) outperformed them all save the latter as expected, but when climbs and dives were incorporated as part of the tests even the Type 95 was outshone. As the Type 96 the new Mitsubishi was officially still under consideration but the urgent needs of the China front overcame caution and the carrier Kaga returned to her home port of Sasebo and exchanged her old aircraft for this newcomer. They soon saw combat.

Among the outstandingly skilled fighter pilots serving with this unit were Lieutenant-Commander Motoharu Okamura, Lieutenant-Commander Minoru Genda himself, Lieutenant Ryoji Nomura and Lieutenant Mochifumi Nangō. One of the group, Petty Officer Isamua Mochizuki, further developed this hineri-komi technique, thus ensuring a fighter aircraft could turn a potentially deadly situation, with a more manoeuvrable enemy aircraft on its tail, into a killing advantage by pulling sharply up into a loop, and, by utilizing a hard left rudder and shifting the control column to the starboard, would side-slip out of the arc prior to reach the top and thereby roll quickly and easily into a spot astern of the opponent in a perfect firing position.¹⁰ The A5M not only had greater speed, climb rate and range than its opponent therefore, but skilled pilots and superior tactics. This combination proved conclusive in the aerial fighting that followed, along with superior numbers (the Japanese outnumbered the Chinese and their foreign ‘advisors’ by three-to-one at this period) and was later also to prove equally deadly to western opponents during the air combats of 1941–42.

Japan’s air ascendency commenced with a devastating assault on 19 September 1937 when Chinese fighters intercepting a carrier-borne dive-bomber attack and, expecting the usual easy pickings, were annihilated by the Japanese escort of twelve A5Ms, which shot down a dozen Chinese machines without loss to themselves. When another raid met Chinese fighters in the same area the next day the Chinese fled without offering any kind of a fight. This established a precedent causing the Army commander to admit that the Navy flyers were as good as his own Army flyers were useless and that the latter might as well be handed over to the former. On 2 December the A5Ms knocked down no fewer than ten Russian I-16Bs without loss and Nanking duly fell to the Japanese by the end of 1937. Meanwhile, aerial fighting moved elsewhere with fierce battles over Nan-ch’ang on two occasions in December when the Chinese lost fifty-four aircraft. The sturdiness of the Mitsubishi design was demonstrated when Petty Officer Kanichi Kashimura’s aircraft lost one third of his port wing when his second victim plummeted into it and sheered this chunk off. Notwithstanding, Kashimura managed to nurse his little fighter safely home. However, a new phase was opened because just to reach this new fighting zone required the Japanese fighters to land and re-fuel at Kuangte en route as the zone was beyond the reach of even the Type 96. To counter the new tactics a fighter with an even greater range was obviously a requirement for the Navy.

The Japanese, with intermittent and half-hearted attempts at negotiation, rejected by the Chinese and elements of their own Army and Government alike, continued to extend their operations even deeper into China, with a new offensive directed toward Hankow to where the Chinese Government had once again retreated. The arrival of Soviet Union aircraft and pilots had helped stem the one-sided nature of the air war for a while and on 29 April 1938 a hard-fought aerial duel took place over the city. Wild claims and counter-claims were subsequently made by both sides.¹¹ Whatever the truth, Hankow went the way of Nanking in October and the Nationalist Government of Chiang Kai-shek, pulled back yet further west up the Yangtze again, this time to the mountain stronghold city of Chungking, on the Upper Yangtze. Air raids duly commenced against this place along with many other Chinese cities from Canton in the south, to Lan-chou in the north-west through which Soviet aid was flowing and Ta’t’ung in the north. In these raids the A5M was impotent as even with wing tanks she could not reach such targets. Unescorted bombers once more began to suffer at the hands of Chinese and Russian fighters and so it continued into 1940. But history was to repeat itself, and once more a breakthrough in Japanese fighter design, even more revolutionary than the introduction of the Type 96, was to turn the tide in Japan’s favour.

The re-equipped and confident Japanese Navy flyers now went about establishing air superiority in the skies over the Chinese capital Nanking toward which the army of General Yasooi Okamura steadily was advancing from June 1938 onward. In October 1938 during the battle of Wuhan the Chinese Air Force was all but eclipsed and withdrawn from combat to re-organize.

In addition to long-range bombing missions the IJN flyers operated close-support missions from Hankow with both dive-bombers and attack bombers but the Type 96 fighters found relatively little employment during 1939. The Japanese Army Air Force aircraft remained relatively inferior but, with German assistance, the Chinese anti-aircraft artillery increased in efficiency. With the introduction of Russian flyers and aircraft, mainly I-15 and I-16 fighters, a resurgence of Chinese fighter defence also began to be once more experienced, with the A5Ms lacking the range once more to provide aerial defence of the G3Ms bombers now joined by the improved Mitsubishi G4M1.¹²

It was clear that a repeat of the earlier situation had been brought about and the Japanese response was the same, the early introduction of a brand-new fighter aircraft. That such a fighter was available at this time was due to two factors, the first being the far-sighted specification laid down by the Navy’s aeronautical planners in calling for a world-beating design, and the second being the skill and perseverance of the Mitsubishi design team in attaining this objective. The whole scheme was truly original and breathtaking in concept. All over the world it was ‘accepted’ that naval aircraft must be inferior to land-based machines. The IJN’s Aviation Department, with the six years’ experience of actual combat in China as their matrix, dared to think otherwise.

The preliminary outlines presented by Lieutenant-Commander Hideo Sawai May 1937 of what was termed the Prototype 12 or 12-Shi¹³ experimental (shisaku) fighter aircraft were revolutionary. But further modifications were incorporated as the latest lessons from China were analysed and these came as a complete surprise to the Mitsubishi and Nakajima design teams when the final Planning Requirement was issued on 5 October 1937.

The overriding requirement laid down for the new aircraft still remained the same, an essential ability to perform – that would ensure the superiority of this fighter over any foreign-built opponent. The Japanese penchant for out-duelling one’s opponent had been the mantra of IJN pilots throughout, and was to remain so. Although this technique had been largely rejected by the other major military air arms since the Great War, Japan remained firmly wedded to this fundamental credo. Everything else was subjected to this ability to out-turn and out-manoeuvre the enemy. Fair enough, this had been the basis of the Type 96, and the 12-Shi was to at least equal, and preferable excel that aircraft’s ability, but now the Navy was demanding more, much, much more. Just how much was revealed once a study was made of the detailed requirements called for by the specification.

The new fighter was to have long range in order to provide the long-range bombers full escort and protection on their missions far into the hinterland of China. But Prototype 12 was still, first-and-foremost, a naval plane and had to operate from carrier decks and be stowed in the ships’ hangar spaces. The Akagi and Kaga were large vessels, reflecting their origins, but the new breed of Japanese aircraft carriers were of smaller tonnages and dimensions, and thus space was at a premium.¹⁴ The limitation placed on the new fighters dimension reflected these carrier space restrictions in that the wing-span had to be less than 12m (39ft 4in). Likewise the length of carrier decks deemed a short take-off length and the new fighter’s limitations in this respect were pegged at less than 70m (229ft 8in) into a wind over deck of 39.37ft/sec (12 m/sec) and with a more generous allowance of 175m (574.2ft) for land operations with no head wind. In a similar manner a landing speed for carrier decks of lower than 58 knots (107km/h) was demanded.

The speed of the Type 96 fighter had proved inadequate once the new foreign aircraft had been introduced in the China war. With the Soviet Polikarpov I-16 Ishak fighters capable of top speeds of 326mph (525km/h), the Type 96’s best performance of 279.62mph (450 km/h) was deemed insufficient. The new fighter needed to attain a top speed that exceeded 310.69mph (500 km/h) at an altitude of 13,123ft (4,000m).¹⁵ For the aerial combat advantages envisaged the Navy planners specified a climb rate of 9,842ft (3,000m) height attainable within 3½ minutes. Altitude fighting required that the new aircraft be fitted with an oxygen inhaler.

Once engaged with the enemy the new fighter would require additional punch to knock down the recently introduced all-metal aircraft of its potential foes. Machine-guns were still deemed essential, but extra hitting power was increasingly seen as vital and so, in addition to a pair of 7.7mm such weapons, the 12-Shi model was to carry two 20-mm Type 99 Model 1, Mk 3, Dai Nihon recoil cannon firing ball, incendiary or high-explosive shells at 450rpm for the first time. This meant additional weight for two sets of 102.294lb (46.4kg), 55 inches long, plus supporting members and the weight of the two thirty-round drum magazines, as did other essential aircraft-carrier operational needs, a Type 96 Ku Model-1 radio set to maintain communication between formations, and also the essential Type 1 Ku Model 3 radio direction-finding outfit to help airborne aircraft re-locate their parent ship. The latter equipment was the standard issue set for most IJN aircraft and had been combat tested by the A5M4. It was known as the Kruesi, Geoffrey Kruesi of Dayton, Ohio being its originator as it was a direct copy of the Fairchild Aero Compass, which Fairchild Aircraft’s Aerial Camera Division was manufacturing in New York and had an American manufactured Eclipse generator. The Fairchild Company’s logo was found on both #3041 at Fort Kamehameha, Pearl Harbor in 1941 and on #4593 that crash-landed near Dutch Harbor in 1942. They were also equipped with a Toyo Electric Corporation crystal-controlled voice or continuous-wave (CW) radio, the Aleutian Zero’s aircraft set being tuned to a 4145 KC frequency.

The vexed question of the reliability of the radio sets fitted in the A6M provoke heated responses and, as with everything about the aircraft, the common fallacy, still being promulgated today, that the Zero pilots simply tore them out and threw them away as they were both useless and weighty. We shall examine this widespread belief in detail later.

Both carrier operations and land-based long-range operations shared the common requirements of range and endurance. In these factors, the 12-Shi was to prove outstanding. The range requirement was 1,161 miles (1,870km) but by fitting an external drop-tank under the fuselage this could be extended to a remarkable 1,932 miles (3,110km). Endurance was to be 1.2 to 1.5 hours at 9,842ft (3,000m) utilizing the normal fuel capacity, extending to two hours with the drop-tank, while it was hoped that by fine-tuning an economical aspiration of between six to eight hours’ flying time could be attained. Additionally, there was even a requirement to carry a pair of 132lb (60kg bombs) in overload condition. The weight of the first prototype when put together, still in a fully complete state, had been 3,452.21lb (1,565.9kg), but a later weighing with full a complement of parts and fuel, came out at 3,570.5lb (1,620kg).

One area of aeronautical expertise in which Japan still lagged behind the more industrialized nations was that of aircraft engine design and construction. This was to be the greatest hurdle of all to overcome for, in demanding all these desirable qualities the Navy was asking that the designers achieve all these advances with a strictly limited range of power plants, none of which exceeded a modest 1,100hp.

Available to the designers of the 12-Shi at this period were just three home-produced twin-row, 14-cylinder, air-cooled radial aircraft engines. Two of these were Mitsubishi products, the first the 875hp Zuisei 13 engine, the alternative the more potent 1070hp Kinsei 64. An alternative to this pair was their rival firm’s Nakajima 950hp Sakae 12. This increased horsepower was achieved with only a 1.3in (3.2cm) increase in diameter and just under 9lb (4kg) increase in weight over the Zuisei. To opt for a rival engine was not a choice Mitsubishi would happily make but it still had to fulfil the demanding specification the Navy was insisting upon. It was a tough ask from a relatively young industry.

The very experienced Nakajima team studied the requirement for three months and then, after a meeting with the naval representatives, as we shall see, decided that it was simply not achievable.¹⁶ They dropped out of the competition leaving the Mitsubishi team as sole contenders. Initially, Dr Jiro Horikoshi’s team had the same reaction. The 12-Shi project at that stage appeared destined to be stillborn.