History
The Hien was notable not just as the sole Japanese fighter in the Pacific War with a liquid-cooled engine, it also fought numerous battles against incoming B-29’s in defense of the Japanese home islands. In December 1940 Kawasaki Aerospace Company produced a prototype 1,175hp engine, Ha-40, a licensed version of the Daimler-Benz DB 601 engine. This liquid-cooled engine was more powerful and had less aerodynamic impact than comparably-sized air-cooled engines. Engine production could not keep up with airframe production so eventually a radial engine was fitted to the airframe and the aircraft designated the Ki-100.