Around this engine Kawasaki planned the Ki-60 fighter, and a lighter aircraft designated the Ki-6 Hien ('Flying Swallow'). The latter was completed in December 1941, and flew well, reaching a speed of 368 mph. During the first half of 1942 the prototype was extensively tested, performing very well against a captured P-40E Warhawk and a German Messerschmidt Bf-109E sent to Japan by submarine.
The submarine also brought 800 Mauser MG151 cannon, which were fitted to the early Ki-61s despite the unreliable supply of electrically-fired ammunition for this weapon.
The Gifu plant delivered 2,654 (or, according to one source, 2,750) of the Ki-61-1 and -1a versions - the latter being redesigned for easier servicing and increased manouevrability. They went into action around New Guinea in April 1943 and were given the reporting-name 'Tony' by the Allies. They were the only Japanese fighters with a liquid-cooled engine.
In 1944 the Ki-61-II was being built, but was only trickling off the production lines, and was suffering from the unreliability of its engine. Moreover the engine was not being produced in sufficient numbers. The initial version of the -II had a larger wing and a new canopy, but it was soon replaced by the -IIa with the older and proven wing. Only 374 of all variants of the -II were built.
In early 1945 one of 275 engineless airframes was fitted with the Ha-112 radial engine. Although a sudden lash-up conversion this produced a staggeringly fine fighter, by far the best ever produced in Japan. This aircraft, designated the Ki-100, was put into production with desperate haste. One of the first Ki-100 units destroyed 14 F6F Hellcats over Okinawa in their first major encounter - without loss to themselves. The easily-flown and serviced Ki-100 fought supremely well against Allied fighters and B-29 bombers to the very end of hostilities in the Pacific.