The Kawasaki Ki-61 Hien (飛燕, "flying swallow") was a Japanese fighter aircraft during the Second World War. The Japanese Army designation was "Army Type 3 Fighter" (三式戦闘機). Its Allied reporting name was Tony and the only mass-produced Japanese fighter of the war to use a liquid-cooled inline V engine. The inline engines used were derivatives of the Daimler-Benz DB series (the Ki-61-I variants used the Ha-40, the Japanese version of the DB 601 engine while the Ki-61-II variants used the Ha-140, the Japanese equivalent of the DB 605 engine) and were generally unreliable due to the Japanese Army having lack of experience with inline engines.
Initially armed with two 12.7 x 81 mm Ho-103 machine guns in the fuselage and two 7.7 x 58 mm SR Type 89 machine guns in the wings, it was upgraded to four 12.7 x 81 mm Ho-103 machine guns (the wing machine guns being replaced), two 12.7 x 81 mm Ho-103 machine guns in the fuselage and two MG 151/20 cannons (20 x 82 mm) in the wings and finally to two 20 x 94 mm Ho-5 cannons in the fuselage and two 12.7 x 81 mm Ho-103 machine guns in the wings (the wing machine guns were sometimes removed).
It was designed by Doi Takeo [土井 武夫] and entered combat for first time in early 1943.
