Why did Japan have technological superiority in aircraft compared to the US at the beginning of the war but was entirely outclassed by the end?

The Japanese aircraft of the early period of the Pacific War were the culmination of a series of long-term trends in its aviation industry and design philosophies. Some of these aircraft were on par with their contemporaries while others were a rather mixed bag where they performed some roles well or better than their American counterpart, but not others. An example of the latter would be the Aichi D3A dive bomber, whose payload and ruggedness was less than the SBD, but it was more nimble and slightly faster than the American dive bomber. The range of the two aircraft were roughly comparable, but the SBD weighed nearly a thousand pounds more and the American plane had a much more robust defensive armament. As the D3A vs SBD example indicates, comparing contemporary aircraft is often complex affair; the D3A possessed some advantages that helped its crew in some scenarios, less so in others. Japanese air superiority in the early stages of the war was due to a variety of factors and cannot simply be boiled down to technological superiority.

The Japanese aviation industry had been painstakingly built up in the 1920s and by the 1930s, it was capable of producing aircraft that incorporate the latest in aviation technology such as variable pitch propellers and flushed rivet construction into its products. In a few areas, the Japanese aviation engineers had surpassed the West. One of the most notable examples of this was the development of ESD that resulted in something very similar to what is known today as 7075 aluminum. The Japanese Mitsubishi company in conjunction with the Sumitomo Metals Company had developed this alloy in secret in 1936 and had bonded aluminum with zinc and chrome to make an aluminum that was much stronger and lighter than other aluminum alloys used by other air forces. They used 7075 for the main spar, typically the heaviest part of an aircraft. 7075 came with a variety of tradeoffs; it was more prone to corrosion over time and much more brittle than other aluminum alloys. This is part of the reason why so few Japanese warbirds can be restored to flying condition, their main spar is too corroded to be safe. But it was only in the postwar period that US firms began producing 7075 in mass quantities. Japanese industry was also able to incorporate light alloys for the skin, which further drove down weight.

Japanese aircraft needed this light aluminum in no small measure because the Japanese aero-engine industry could not deliver powerplants with enough power to adequately service a heavier airframe. Japan had begun to develop domestic radial engines on par with those of the West by the late 1930s, but lagged significantly behind in liquid-cooled engines. The Japanese licensed-production of the German DB-601, the Aichi Atsuta and Kawasaki Ha40, suffered from perpetual bugs as Japanese industry could not produce the fine tolerances for the long crankshaft of an inline engine in mass quantities. The radials Japan produced by1940, the Mitsubishi Kinsei and Nakajima Sakae, were compact and reliable engines that produced a respectable amount of horsepower. But the 1940 generation of Japanese engines were unable to incorporate the latest developments in aero-engine technology very well and Japanese R&D for engines was constrained by a narrow technical base and competing industrial firms loath to share technical secrets. The interservice rivalry compounded these matters as the IJA and IJN favored some firms over others and prevented standardization and diffused the few resources available to Japan.

The Japanese pilots in 1941 and 1942 also had the benefit of both experience and doctrine on their side. The China War had proved to Japan of the necessity of bomber escorts, so both the IJA and IJN had drop-tanks and a coherent escort doctrine. Just as the Spanish Civil War had imparted to the Germans the lesson that WWI tactics like the vic were obsolete, the air battles with the RCAF had allowed Japan to develop a set of fighter tactics that suited modern air warfare much better than tight-vees. More importantly for the IJN, the interwar period had allowed for the working out of carrier doctrine in which a carrier division would launch airstrikes in a series of coordinated pulses. The composition of the Pearl Harbor attack force illustrates this. The 1st and 2nd Carrier Divisions (Akagi & Kaga, Hiryu & Soryu) were able to launch the bulk of their B5Ns while the less experienced 5th Carrier Division (Zuikaku & Shokaku) launched their D3As for the first wave, and the composition was reversed for the second wave. Thus the first wave of the attack concentrated its heaviest-hitting aircraft with its most experienced air groups while the second wave allowed the less-experienced B5Ns of the 5th Carrier Division to attack soft targets while the D3As of the 1st and 2nd Divisions would attack targets hit by the first group. This type of pulse attacks were difficult to coordinate on the fly and made Japanese carrier operations unwieldy, but they maximized the carriers' airpower when they worked. In contrast, American carrier strikes in the early war period were uncoordinated and made inefficient use of their carriers' air wing. Both the mass shootdowns of the torpedo squadrons and the lopsided victory of the SBDs at Midway obscures the fact that the American strikes were chaotic and could have allowed the Japanese to deal with the attacks piecemeal. Fortunately for the USN, one area in which Japanese carrier doctrine was really behind was in its CAP and air defense.

Thus while Japanese aircrew flew decent aircraft of roughly homogeneous quality and with a consistent doctrine in 1941 and 1942, the same could not be said of their American counterparts. American aircraft in 1941 were in the midst of reequipping with more modern aircraft, so some units had to meet the Japanese with semi-obsolescent planes like the Vindicator or Devastator bombers. Moreover, the rush to give American aircraft more modern equipment coupled with the need to reinforce East Asia created a logistical nightmare for American aircrew facing the Japanese. In the case of the Philippines, the reinforcement of the archipelago with P-40s caused maintenance headaches as the planes often lacked necessary equipment such as gun chargers and shortages of ammunition. Additionally, the American aircrew had yet to come up with a clear set of air doctrines to meet the Japanese onslaught. So not only were the Americans operating in conditions of numerical inferiority, they had to learn on the job. In the case of the Philippines and Indonesia, this learning curve was incredibly steep. It was not until 1942/43 that the early war lessons began to sink in and the newer generation of aircraft began to be fielded. Tactics like the Thatch Weave denuded Japanese turning tactics of much of their efficacy and the American training establishment began churning out pilots of good quality in numbers Japan could not match.

In terms of raw technological advantages, the Japanese aviation industry simply could not compete with America. While the 1940 generation of Japanese aircraft compared well to their American contemporaries, the American aviation firms were only just starting to get their first wind in this period. Unlike Japan, America did not have to rely upon ersatz materials to make up for shortages and US R & D already had a leg up on supercharging, high octane avgas, and radial engines that made the mid- and late- war American aircraft far more powerful than subsequent Japanese developments. The replacement engine for early war generation engines, the Nakajima Homare, delivered a little less than 2000 hp (assuming decent quality fuel), while the American Pratt & Whitney R-2800 was delivering an excess of 2000 hp reliably by the mid-war period. The growth in airframe weight that was common in all air forces in this period meant that Japanese designers had to do more with less powerful engines.

But not all of the problems confronting the mid- and late-war Japanese air effort were material in nature. There was substantial human wastage as neither the IJN nor the IJA were able to revamp their training regimens to meet the necessities of attritional warfare. The prewar training regime was designed to create a corpus of highly-trained pilots and weeding out the inferiors for minor infractions (many pilot memoirs from this period describe training more like hazing than learning practical skills), but this system was ill-suited for wartime. Japanese industry's subcontracting process was in the words of the US Strategic Bombing Survey, "badly planned and poorly organized." There was some effort to disperse air production between 1941-1944, but the start of American firebombing forced an even more hasty dispersal program that ate into dwindling efficiency. Interservice rivalry continued and undermined attempts to rationalize the system of procurement and production. Although American production numbers swamped Japanese ones, there was one area in which Japan remained competitive with their enemy: the number of aircraft types fielded. The Materials Planning Board, which was supposed to coordinate industry with the services was never able to reign in the military's demands for specialized aircraft.