The Japanese Army found itself faced with the prospect of 275 Ki-61-II airframes sitting around waiting for installation of their Ha-140 liquid-cooled engines. The Ha-140 engine had proven to be totally unreliable and to make matters worse, the factory manufacturing the Ha-140 had been destroyed in a B-29 raid. Since Japan desperately needed aircraft capable of intercepting the B-29's, in November of 1944 the Ministry of Munitions instructed Kawasaki to install a different powerplant in the Ki-61-II in an attempt to get as many aircraft in the air as possible.

After some sniffing around, Kawasaki finally settled on the 1500 hp Mitsubishi Ha-112-II fourteen-cylinder double-row radial engine. This engine had established a standard of easy maintenance and reliable service, which contrasted markedly with the notoriously unreliable and temperamental Ha-140. However, the Ha-112 was a radial engine, and, with a diameter of four feet, the installation of this engine in a fuselage only 33 inches wide provided a major challenge. However, the Kawasaki concern was guided in its work by being able to study the engine mount in an imported Focke-Wulf Fw 190A, an example in which a wide radial engine had been successfully installed in an airframe with a narrow width. In addition, the same Mitsubishi Ha-112 radial engine had been successfully installed in the Aichi-built D4Y3 (Allied code name JUDY) dive bomber, earlier versions of which had been powered by a liquid-cooled engine.

The new project was sufficiently different from the Ki-61 Hien that it was assigned a new Kitai number: Ki-100. Three Ki-61-II airframes were experimentally modified as Ki-100s by the installation of the Ha-112 radial. The first Ki-100 prototype aircraft made its first flight on February 1, 1945. The results of the flight testing exceeded everyone's expectations. The Ki-100 was about 600 pounds lighter than its Ki-61-II predecessor. Maneuverability and handling were markedly improved due to the lower wing and power loading. Although the maximum speed of the Ki-100 was slightly lower than that of the Ki-61-II because of the higher drag exerted by the radial engine, this performance could be reliably attained because of the better reliability of the Ha-112 engine. The design was ordered into immediate production as the Army Type 5 Fighter Model 1A (Ki-100-Ia). The first Type 5 fighters (Ki-100-Ia) were direct conversions of existing Ki-61-II airframes. 271 airframes were converted between March and June 1945, and were immediately delivered to operational units. The Ki-100 was simple to fly and maintain. Even the most inexperienced pilots were able to get the hang of the Ki-100 relatively quickly. The Ha-112 engine proved to be quite reliable and simple to maintain. In combat, the Ki-100-Ia proved to be an excellent fighter, especially at low altitudes. It possessed a definite ascendancy over the Grumman F6F Hellcat. In one encounter over Okinawa, a Ki-100-equipped unit destroyed 14 F6F Hellcat fighters without loss to themselves. When the Ki-100 encountered the P-51D Mustang at low or medium altitudes over Japan, it was able to meet the American fighter on more or less equal terms. The outcome of P- 51D vs Ki-100 battles was usually determined by piloting skill or by numerical advantage rather than by the relative merits of the two fighter types. However, at altitudes above 26,000 feet, the maneuverability of the Ki-100 began to fall off rather severely and the fighter was at a relative disadvantage in intercepting the high-flying B-29.

The Ki-61-I KAIc went into production in January 1944, and ultimately replaced production of all earlier models in August 1944. The Ki-61-I KAIc would become the heavily-produced version of the Hien (Swallow), accounting for over half the total number built. A few Ki-61-I KAId bomber interceptors were also built in late 1944. These variants incorporated two 12.7 millimeter guns in the fuselage and a 30 millimeter gun in each wing. So far as I am aware, the Ki-100 never had a separate Allied code name assigned to it. It may, for all I know, have been known under the code name of its predecessor --- TONY.

By June, 1945, all of the Ki-61-II airframes had been used up, and further Ki-100s were built from the outset as radial-powered machines. This version was designated Ki-100-Ib. The Ki-100-Ib differed from the Ki-100-Ia in having an all-round vision hood similar to that fitted to the experimental Ki-61-III. The first Ki-100-Ib fighters were built at the Kagamigahara and Ichinomiya Kawasaki factories in May of 1945, but production was severely hampered by the continual Allied bombing. Plans had been made to produce 200 fighters per month, but the Ichinomiya plant was forced to shut down in July 1945 after having built only 12 aircraft, and the Kagamigahara plant had its production severely curtailed by aerial attacks. By the time of the Japanese surrender, only 118 Ki-100-Ib aircraft had been delivered.

In an attempt to improve the high-altitude performance, the Ki-100-II version was evolved. It was powered by a 1500 hp Mitsubishi Ha-112-II Ru with a turbosupercharger and water-methanol injection to boost power for short intervals. Because of a lack of space, the turbosupercharger had to be mounted underneath the engine without provision for an intercooler and its associated ducting, with air being ducted directly from the compressor to the carburetor. It first flew in May 1945. The lack of an intercooler limited the high-altitude performance of the Ki-100-II, and the turbosupercharger added 600 pounds to the weight, which reduced maximum speed by 15 mph at 10,000 feet. However, the boosted high-altitude power enabled a maximum speed of 367 mph to be be reached at 32,800 feet (the cruising altitude of the B-29 during daylight operations). It had been planned to begin production of the Ki-100-II in September of 1945, but only three prototypes of this high-altitude interceptor had been produced by the time of the Japanese surrender.

A total of 396 Ki-100s were built, including 275 Ki-61-II conversions, 118 Ki-100-Ib production aircraft built from scratch, and three Ki-100-II prototypes. Most of them were assigned to the defense of the home islands, operating from Chofu and Yokkaichi from the spring of 1945. At the end of the war, two Ki-100-Ibs were shipped to the USA for evaluation. I don't know what happened to these planes. Presumably, they were scrapped in the late 1940s, along with a lot of other captured Axis aircraft.