The Kawasaki Ki-100 (キ100) is a single-seat single-engine monoplane fighter aircraft used by the Imperial Japanese Army Air Service during World War II. The Japanese Army designation was . It was not assigned an Allied code name. 275 Ki-100s were modified from Ki-61s as an emergency measure to accept a 14-cylinder Mitsubishi Ha-112-II radial engine in place of the original Kawasaki Ha-40 inverted V-12 inline engine which resulted in one of the best interceptors used by the Army during the war. It combined excellent power and maneuverability, and from the first operational missions in March 1945 until the end of the war, it performed better than most IJAAF fighters against both United States Army Air Forces Boeing B-29 Superfortress bombers and North American P-51 Mustang fighters as well as U.S. Navy Grumman F6F Hellcat carrier fighters. A newly built variant, the Ki-100-Ib, was produced with a cut down rear fuselage during the last months of the war which equipped five home defence sentai. High-altitude performance was further improved with the final variant, the Ki-100-II, however only three of these were produced before the war ended and this final variant never saw operational service. Design and development The Ki-100 was a stressed-skin cantilever low-wing single-seat enclosed-cockpit radial engine monoplane fighter with retractable undercarriage. Control surfaces were fabric covered. Ki-61 engine problems In mid-1944, the Ki-61 was one of the best fighters of the Imperial Japanese Army Air Service. It was also the only production Japanese fighter to have an inline powerplant, the V-12 Kawasaki Ha-40, a Japanese adaptation of the German Daimler-Benz DB 601 engine, as well as one of the first with factory-installed armor and self-sealing fuel tanks. It also had a respectable performance, in line with contemporary American designs, with speed and rate of climb emphasized instead of manoeuvrability and range. It was an effective design, but suffered from engine shortages and reliability problems. These problems led to the development of an improved model, the Ki-61-II (later Ki-61-II-KAI), powered by the improved Kawasaki Ha-140 inverted V-12 engine, which was heavier than the Ha-40 it replaced. Maximum speed increased from and aside from the rate of climb, general performance was improved as well. However, it never performed as expected due to continued quality control problems with the engine, while far fewer engines were produced than required. Ha-112 Radial engine At this point of the war, the IJAAF was in desperate