The Kawasaki Ki-61 Hien (??, "flying swallow") is a Japanese World War II fighter aircraft used by the Imperial Japanese Army Air Service. The Japanese Army designation was "Army Type 3 Fighter" (?????).[2] Allied pilots initially believed Ki-61s were Messerschmitt Bf 109s and later an Italian aircraft, which led to the Allied reporting name of "Tony", assigned by the United States War Department.[3] It was the only mass-produced Japanese fighter of the war to use a liquid-cooled inline V engine. Over 3,000 Ki-61s were produced. Initial prototypes saw action over Yokohama during the Doolittle Raid on 18 April 1942, and continued to fly combat missions throughout the war.