Think of Japanese planes of WW2, and most will think of the Mitsubishi A6M Zero, and that it was the finest fighter plane in the war.
First of all, not all Japanese fighter planes were Zeros. The Japanese Army and Navy had their own air forces, with their own planes. At the start of the war, the Army used mostly the old model Nakajima Ki-27 and was just introducing the somewhat more advanced Ki-43 Hayabusa, also manufactured by Nakajima. The Invasion of Malaya in early 1942 was supported by Army airfields based in Thailand, and various sources say the old Ki-27 was the most numerous plane in the Burma theatre, with a sprinkling of Hayabusas.
Secondly, the A6M Zero was NOT the best fighter of the War. By mid-1942 the weakness of the Zero became known. It was superbly maneuverable, and better armed than the Army’s planes. But it wasn’t actually very fast. It’s engine wasn’t really very powerful and it didn’t climb very well, and power dropped at higher altitude. And lastly, it was fragile. To save weight and maximize range, Mitsubishi threw away safety features such as armour and self-sealing fuel tanks, making the fighter quite flammable after a few hits. The light weight also made the Zero slow in a dive.
This design flaw was also present in early Army planes, with the same consequences: Allied fighters learned not to slow down and engage in maneuvering-intensive dogfights. They would exploit their own fighters’ excellent climb, diving speed and heavier armaments, and simply zoom in from above, fire a burst and zoom away. Zeros and the Ki-27 and Ki-43 were fragile enough that an en passant burst of fire was enough to destroy them. If attacked by Zeros, the Allied fighters would simply dive and run away.
It wasn’t even the best plane the Japanese had! The Zero was better than the Ki-27 and Ki-43, but later the Army introduced Nakajima’s Ki-84 Hayate “Storm”, an excellent fighter. It was better armed, and above all, it had armour protection. It did not burst into flames from just a few hits. It had a new powerful engine, the “Homare”, which in good condition enabled the fighter to outrun the Mustang.
The Navy commissioned a new plane, the A7M “Reppu” but imposed a constraint on the design team: it must use the existing Homare engine instead of a more powerful engine still in development. The prototype proved to be underpowered and unsatisfactory: the Reppu, intended to have armour and heavier weapons, was larger and heavier than the Hayate. The Homare engine did not suffice. In the end, no Reppus were produced. The Navy largely soldiered on with its obsolete Zero…
…except they did receive an unexpected new fighter from Kawanishi, a company that specialized in seaplanes and flying boats. The Kawanishi N1K Kyofu was a single-engined seaplane, whose huge floats prevented it from flying very fast. Kawanishi discovered it was a pretty good land-based fighter with the floats removed. The plane, minus its awkward floats and given the Homare engine, now had enough power to fly at high speed, had excellent climb and dive, and could carry four 20mm cannons, very heavy armaments by Japanese standards. It was also very robust, and could take a beating, unlike its fragile counterpart the Zero. As a land-based fighter, it was now called the N1K1-J Shiden, “Purple Lightning”. Further modifications gave the Shiden-Kai “Improved Purple Lightning” even better performance, but the war was nearing its end by this point. Only a few hundred Shiden-Kai were made, but most pilots agree it was one of the best fighters of the war.
Although, I suspect the Japanese Army would say their planes were still better. The Ki-84 Hayate has been mentioned, but the Kawasaki Ki-61/100 deserves attention. Early in the war, both Army and Navy, with aircraft such as the Ki-27, Ki-43 Hayabusa and A6M Zero, emphasized maneuverability and long
range, at the expense of speed, protection and sometimes heavy
armaments, (The Army’s Ki-43 Hayabusa only had two light 7mm machine
guns). The Ki-61 Hien (Flying Swallow) was an experimental idea the Army played with, a fighter that emphasized speed and protection and weapons, at some expense to maneuverability. Kawasaki had a German consultant in the company in the 1930s, who kept insisting on the superiority of water-cooled inline engines and the slim streamlined designs they made possible.
Kawasaki borrowed German engine design. Instead of the air-cooled radials more typical of Japanese aviation engines, the Ki-61 was powered by a slightly modified version of the same water-cooled engine as the German Messerchmitt Bf-109. The use of a water-cooled inline engine enabled the Ki-61 frame to be built with a narrow, and better streamlined fuselage, rather than the fatter look typical of all single engined aircraft powered by radial air-cooled engines.
Radial engines have their cylinders arranged in a ring, with the crankshaft it the centre, and the engine has to be mounted with the ring of cylinders perpendicular to the airflow, taking advantage of the slipstream for cooling. An aircraft powered by a radial has a wider cross-section and greater frontal drag. Aircraft powered by liquid cooled inline engines can be made with a more streamlined nose and slimmer fuselage, for better performance. On the other hand, the necessity of managing liquid around the engine and its radiator made liquid-cooled engines typically heavier for the same amount of power, and more sensitive to damage, more complex to produce and can be harder to maintain.
The Ki-61 Hien was the fastest plane the Japanese had early in the war, but the engine, modified to shave off 30kg from its German original, was extremely complicated to produce and maintain. It was constantly breaking down, especially in the tropics, and being liquid-cooled with a radiator system, more sensitive to damage than a robust air-cooled radial engine. It was kept in production mainly because of its good high-altitude performance, which became increasingly more important as Allied bombers started appearing over Japan.
As it chanced, a bombing raid destroyed the engine factory, leaving hundreds of completed Ki-61 airframes without engines. An experimental mating of the airframe with a small-diameter radial was attempted and proved surprisingly successful. The air cooled radial engine was more than 300kg lighter, and gave the fighter new characteristics, making it even more nimble than before with the same power. The resulting fighter, the Ki-100, actually outperformed the Army’s stalwart Ki-84 Hayate. It wasn’t given a nickname, but regarded as sufficiently different that it was designated Go-shiki-sen, Army Fighter Type 5. By this point, however, the war was in its final stages.
We can see that in the later stages of the War, it was the Army that had the better fighters, mostly Nakajima Ki-84 Hayate, with a notable entry by Kawasaki with its Ki-61 and Ki-100. The Navy had an peculiar saviour in Kawanishi’s converted seaplane, the N1K Shiden, but the famed Zero was largely obsolete by 1943.
P.S. One surprising little fact: Nakajima was licenced to build Zeroes, and ended up building more of them (6500) than Mitsubishi (about 4000).
Japan had a number of companies building aircraft, the ones mentioned in this post are: Mitsubishi, Nakajima, Kawasaki and Kawanishi. There were others.