Kawasaki purchased a license to build the German DB 601engine in 1937. The engine was revised and became the HA-40. Built around this engine was the Ki-61 Code named "Tony" by the allies. These planes entered service around New Guinea in April of 1943 and flew well. This was the only Japanese aircraft designed with a liquid cooled engine. Although not supplied in large numbers the Tony flew constantly but slowly started to fade through battle attrition and lack of spare parts.
Later in the war the Tony was assigned to intercept Allied bombers as they pounded Japan. In this mission the Tony ran up against the superior P51 Mustang were they took a beating.