The Kawasaki Ki-64 was a radical design for a high-speed single-seat fighter powered by two engines both mounted in the main fuselage. The basic idea for the Ki-64 was produced by Takeo Doi, head of Kawasaki's design team, in 1939. Externally it resembled a conventional single engine single-seat fighter, but it was actually powered by two engines, one mounted in front of the cockpit and one behind. Detailed design work on the project didn't begin until October 1940. Doi adopted the Kawasaki Ha-201 engine, which despite its name was actually two Ha-40 licence built DB601A engines, each powering one of the contra-rotating propellers. The rear engine powered the controllable-pitch forward propeller, while the forward engine powered the fixed-pitch rear propeller.
Engine cooling was provided by a steam vapour system which used surface condensers mounted on the wings and flap surfaces, combined with a small retractable radiator for use while on the ground. This system was initially tested on a Ki-61-I, before being installed in the only Ki-64 prototype. The wing based cooling system reduced drag, increasing speed by 25mph, and was less vulnerable to damage, but it also reduced the amount of space available in the wings for fuel tanks.
The prototype was completed in December 1943. On the fifth test flight the rear engine caught fire, forcing an emergency landing. The engines and the airframe were separated for repairs, and never reunited. A modified design, the Ki-64 KAI, was suggested, powered by a 2,800hp Ha-201 engine and using electrically operated constant speed contra-rotating propellers. This second design was never built.
Engine: One Kawasaki Ha-201 ([Ha-72] 11) twenty-four cylinder inverted-vee liquid cooled engine made up of two Ha-40 engines paired
Power: 2,350hp at take-off, 2,200hp at 12,795ft
Propellers: Three-blade contra-rotating propellers
Wing span: 44ft 3 1/2in
Length: 36ft 2 1/4in
Height: 13ft 11 5/16in
Empty Weight: 8,929lb
Loaded Weight: 11,244lb
Max Speed: 429mph at 16,405ft
Service Ceiling: 39,370ft
Range: 621 miles
Armament: Two 20mm Ho-5 cannon in fuselage decking, two 20mm Ho-5 cannon in wings