The Kawasaki Ki-100 was designed to combat against the B-29s that were flying at 30,000'.
The Kawasaki Ki-100 was made by combining Ki-61-II airframes with the Mitsubishi Ha-112-II radial engine.
The designers of the Ki-100 studied the engine mounting system on the Focke-Wulf 190A to determine a method for mounting the small diameter Ha-112 radial.
The Kawasaki Ha-140 engine was to be used but the Akashi factory was destroyed in an air raid.
As there were airframes waiting to have engines installed, Kawasaki converted three by installing the Mitsubishi Ha-112-II engine. Once these were show to be successful the rest of the airframes were converted by May 1945.