The Kawasaki Ki-61 Hien was developed in response to a 1939 Japanese Army requirement for a modern, liquid-cooled fighter built around the Daimler-Benz DB 601 inline engine. Kawasaki secured a license to build the engine as the Ha-40 and began work on two designs: the interceptor-oriented Ki-60 and the more versatile Ki-61. The Ki-60 proved disappointing, but the Ki-61 incorporated lessons from its failure, including a lighter airframe, better maneuverability, and improved balance between speed and agility. The first prototype flew in December 1941, only weeks after Pearl Harbor. The Ki-61 stood apart from other Japanese fighters, most of which used air-cooled radial engines. Its sleek, liquid-cooled engine and narrow fuselage caused Allied pilots to mistake it for a German or Italian design, leading to its Allied code name “Tony.” Test pilots praised its self-sealing fuel tanks, upgraded armament, and dive performance. In trials, the Ki-61 outpaced contemporaries like the Ki-43 Hayabusa and even compared favorably with captured Western fighters. However, skeptics in the Army worried about its higher wing loading, which made it less agile than the lightly built fighters they were used to. Production began in 1942, and the Ki-61 entered combat that April during the Doolittle Raid over Japan. Standard armament included two fuselage-mounted 12.7 mm machine guns and wing-mounted weapons, with some later models carrying heavier 20 mm cannon. Unlike earlier Japanese fighters, the Hien offered pilots armor protection and self-sealing tanks, making it tougher in combat against American aircraft such as the P-40 and later the P-51 Mustang. Still, the Ha-40 engine proved temperamental, requiring precision manufacturing and struggling in tropical conditions. Reliability issues plagued frontline units, especially in New Guinea and the Solomon Islands. To improve performance, Kawasaki developed the Ki-61-II with a larger wing and the more powerful Ha-140 engine, rated at 1,500 hp. While this version promised higher speed and better altitude capability, the Ha-140 suffered from severe reliability problems. Many engines failed in service, and a U.S. bombing raid in January 1945 destroyed the main factory producing them. Hundreds of unfinished Ki-61-II airframes were fitted instead with Mitsubishi Ha-112 radial engines, becoming the Ki-100, which proved more reliable though less effective at high altitude against B-29 bombers. In total, more than 3,000 Ki-61s were built. Although hampered by engine problems and overshadowed by later designs, the Hien was Japan’s only mass-produced liquid-cooled fighter and represented an attempt to field a sturdier, more heavily armed aircraft. Its service from 1942 through the end of the war reflected both Japan’s technical ambitions and the industrial difficulties that limited their realization.