The dramatic advances made in aeronautics after WWI gave rise to many innovative designs that led to new uses for airplanes in naval combat.
Many countries which were surrounded by water began development of planes fitted with floats which could take off from water or be launched from catapults on warships. They were used for reconnaissance and in a defensive role against surface and submarine vessels, with the opinion being that these were the limits of floatplanes. The only country to use floatplanes more widely was Japan. Although the idea wasn’t new and had been attempted by other countries, the Japanese were the first to design and mass produce a float-equipped fighter and use it in combat.
During the attack on China the Japanese naval air forces were very successful at quickly establishing air superiority. A side effect of the invasion, however, was to convince the Japanese command of the need to ensure access to strategic natural resources such as crude oil, rubber and iron ore if they were to continue waging war. Those resources were readily available on the islands of the southwest Pacific, and would be easy enough to capture, but doing so would put Japan at war with Great Britain, Holland and the United States. It was clear to the Japanese command that once islands were captured they would serve as bases for further attacks and airfields would need to be built. On the larger islands in the region the Allies already had airfields. Taking this into account, the Japanese planned to use the smaller islands and atolls in less protected regions for their airfields as well. The sheer size of the arena at hand limited the use of aircraft carries to a few operations at once, and it was imperative that amphibious groups have air cover for landing operations, even if land-based airfields weren’t ready yet. Some kind of defense was also needed while the fleet was carrying on offensive maneuvers in other areas of the Pacific. The “15-Shi” specification for a float-equipped fighter was laid out by Kaigun Koku Hombu (Navy Air Headquarters) in September, 1940 for use on atolls and small islands that were too small or the terrain too unfriendly to support an airfield. It was to be a single-engine, single-seat fighter powered by a single radial engine with one main float and two helper floats. The contract was awarded to Kawanishi in the hope that they would be able to quickly come up with a design. Kawanishi assigned the N1K1 project to a group of engineers including Toshihara Baba, Shizuo Kikuhara, Hiroyuki Inoue and Elizaburo Adachi. Although they were very enthusiastic about the project the prototype turned out to be difficult to build. The engineers planned to use a powerful 1,460 hp Mitsubishi engine that would turn two contra-rotating two-bladed propellers but had problems with the gear box. Kaigun Koku Hombu was getting more impatient as war drew closer, and the final version of the plane wasn’t even on the drawing table yet. Over a year and a half had passed when the contra-rotating propellers were finally removed in favor of a single three-bladed propeller. The N1K1 prototype flown in May, 1942 flew well but Kawanishi’s production strength was too small to put the plane into full-scale production. A total of eight prototypes and 89 production models were built.
In the meantime, Kaigun Koku Hombu was pressed for time and decided to examine the possibility of adapting an existing fighter to a floatplane. At the time the best fighter in the Imperial fleet was the recently introduced Mitsubishi A6M2 Model 21 which had literally hammered the enemy competition in China. The decision to make a floatplane version of it according to the “15-Shi” specifications and modifications included in “16-Shi.” At that time Mitsubishi was working at full capacity on other projects, so the decision was made to subcontract the work to Nakajima Hikoki K. K. in Ota, where they had just started making the Zero under license, and the design offices had the necessary resources available for the project.
In February, 1941 two engineers, Niitake and Tajima started work on a project called AS-1. Nakajima had been building aircraft for the Japanese army, and had come up with successful fighters such as the Ki-27 or Ki-43. A very successful shipboard torpedo bomber, the B5N was designed for the Navy, but with a traditional undercarriage. So the project wasn’t going to be easy, and the engineers would have to study the dust-covered notebooks of their predecessors.
After examining different possibilities, it was decided that the A6M2 would be an ideal candidate for floats, with little change necessary needed for the conversion. The main work was in designing the floats and how best to mount them. The result was an almost unchanged original with added floats called the A6M2-N.