The Kawasaki Ki-61 Hien (飛燕, "flying swallow") was a Japanese World War II fighter aircraft used by the Imperial Japanese Army Air Force. The first encounter reports claimed Ki-61s were Messerschmitt Bf 109s: further reports claimed that the new aircraft was an Italian design, which led to the Allied reporting name of "Tony", assigned by the United States War Department. The Japanese Army designation was "Army Type 3 Fighter" (三式戦闘機).It was the only mass-produced Japanese fighter of the war to use a liquid-cooledinline V engine. Over 3,000 Ki-61s were produced. Initial prototypes saw action over Yokohama during the Halsey-Doolittle Raid on 18. April 1942, and continued to fly combat missions throughout the war.