In 1940, anticipating a coming war in which Japan would be involved in far-ranging offensive operations, the Japanese Navy issued a requirement for a floatplane fighter which could be used for offensive operations in forward areas where no airfields existed. In response to this requirement, Nakajima offered a design based on a modified Mitsubishi A6M2 Reisen ("Zero") carrier-based fighter. This eventually emerged as the A6M2-N and was given the Allied code name "Rufe" when it entered service. However, this aircraft was considered as only an interim solution, and Kawanishi was instructed to begin work on a more advanced aircraft specially designed for the purpose.
A team of engineers at the Kawanishi plant came up with a design for a compact aircraft with mid mounted wings of laminar flow section. A single large float was to be installed underneath the fuselage with auxiliary floats being carried underneath each outer wing. The central float was to be attached to the fuselage by a forward V-strut and an I-strut at the rear. The initial design had the auxiliary floats being retractable and with metal planing bottoms and inflatable rubberized fabric tops. However, these retractable floats were deemed to be too heavy and complex and were replaced by fixed cantilever floats prior to the first flight of the prototype. The aircraft was to be powered by a 1460 hp Mitsubishi MK4D Kasei 14 air-cooled radial driving a pair of counter-rotating two-bladed propellers. The counter-rotating propellers were intended to offset the high propeller torque on takeoff expected from such a powerful engine mounted in a relatively small airframe.
The first N1K1 prototype took off on its first flight on May 6, 1942. The counter-rotating propellers of the prototype were later dropped as being too complex, and the design was modified to accept a 1460 hp MK4C Kasei 13 (Ha.32/13) radial engine driving a single conventional three-bladed propeller via an extension shaft. The single propeller installation was simpler and less mechanically troublesome, but it did produce (as expected) an extremely powerful torque on takeoff that required considerable skill on the part of the pilot to counter.
Once in the air, the N1K1 was found to be an extremely pleasant aircraft to fly and it had remarkable maneuverability by virtue of its combat (Fowler) flaps. The Japanese Navy accepted the design under the designation N1K1 Navy Fighter Seaplane Kyofu (Mighty Wind) Model 11, and production began.
Service trials aircraft were delivered to the Japanese Navy starting in August of 1942. 1460 hp MK4C Kasei 13 engines early production aircraft. 1530 hp MK4E Kasei 15 engines, differing only in minor details, powered later production aircraft.
The Kyofu entered service with the Japanese Navy in July of 1943 and was assigned the Allied code name "Rex". Production was slow in gaining tempo and by December of 1943 had reached only 15 per month.
However, by the time that the Kyofu entered service, Japan had been thrown back onto the defensive, and the Kyofu was never to serve in the offensive fighter role for which it had been designed. Instead, the N1K1 was assigned as an interceptor based at Balikpapan in Borneo, a role for which it had never been intended. Even though the Kyofu was a rugged and efficient floatplane, it was no match for the single-seat Allied fighters which opposed it. Consequently, production of the Kyofu was terminated in March of 1944 after the delivery of only 89 production aircraft. Also abandoned at the same time was the N1K2-I Kyofu-Kai project, to have been powered by the improved 1900 hp Mitsubishi MK4R Kasei 23 engine.
Later in the war, one Kyofu unit was assigned as an interceptor with the Otsu Kokutai operating from the inland Lake Biwa on the Japanese home island of Honshu.
The National Museum of Naval Aviation of Pensacola, Florida currently owns an N1K1. It had been brought to the United States after the war and had sat out in the open for many years at NAS Willow Grove, Pennsylvania before being moved to more secure quarters at Pensacola. Another N1K1 is in storage at the NASM Garber Restoration and Storage Facility at Silver Hill, Maryland. It awaits restoration and display.