In 1939 Kawasaki’s chief engineer, Takeo Doi, conceived the draft of a high-speed unorthodox fighter while he was working on the development of the Ki-45 and Ki-61. By that time the Japanese Army was in need of more conventional aircraft and did not authorise to proceed with this aircraft until October 1940, when the project was revived under the Ki-64 designation to meet a specially issued specification for a maximum speed of 435 mph at 16,405 ft and a climb to 16,405 ft in 5 minutes.
Takeo and his colleagues of the Akashi plant decided to use the 2,350 hp Kawasaki Ha-201 which in fact was made out of two Ha-40 twelve-cylinder liquid-cooled engines mounted in tandem fore and aft of the pilot’s cockpit and drove two contra-rotating three-blade propellers. The forward propeller was driven by the rear engine and was of controllable-pitch type. The rear propeller, driven by the forward engine, was of fixed-pitch. The most unusual feature was the steam vapour cooling system, that used the wing and flap surfaces for cooling area. The coolant was water carried in two tanks in the wings. The front engine used the cooling elements of the port wing and the rear engine those of the starboard wing. A Ki-61 was specially modified to test the entire powerplant in a wind-tunnel. These engine tests delayed the completion of the prototype until December 1943, but they showed that the cooling system performed satisfactorily. The engine speed in 25 mph but it also left limited space for fuel tanks, thus reducing range.
Only five flight tests were made since December 1943. One of the engines failed in the fifth test and the aircraft made an emergency landing. The engine repair was never completed and the airframe was captured by American troops at the end of the war and some elements of the cooling system evaluated in the U.S.
The Ki-64 Kai as a propose production version with a more powerful Ha-201 engine rated at 2,800 hp at altitude and driving electrically-operated constant-speed contra-rotating propellers. A maximum speed of 497 mph was anticipated, but the project was cancelled in favour of more conventional types already in production.