@Mulboyne:
National Museum of Nature and Science has discovered footage of the KEN III the Kawasaki Ki-78 research aircraft project, which was renamed the Kawasaki Ki-78 when taken over by the military during WWII. Apparently, only two prototypes were ever built.
Besides the Kawasaki Ki-78, Japan also had working versions of the following according to WW2 Aircraft Ranked-by-Speed website::
The 712 kph/442 mph Nakajima Ha-211 an 18-cylinder air-cooled radial piston type - while driving a twin-propeller arrangement: there was a four-bladed propeller unit at the nose and a second unit at the extreme end of the fuselage nacelle driven via an extension shaft (termed a "push-pull" configuration). Four working prototypes of the Mitsubishi Ki-83 Twin-Engine Heavy Fighter could reach 705 kph/438 mph . Nakajima Ki-87 High-Altitude Fighter / Interceptor capable of 700 kph/435 mph.