The Kawasaki Ki-61 Hien (飛燕, roughly "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-cooled inline V engine. Over 2,500 Ki-61s were produced, first seeing action around New Guinea in 1943, and continuing to fly combat missions throughout the war.
Source: Wikipedia, "Kawasaki Ki-61", available under the CC-BY-SA License.
As Described In
Aces of the Pacific: Kawasaki mated a native design to a German liquid cooled engine to create the Ki-61 Hien. It first reached the front in mid-1943, and quickly established a reputation as a dangerous, fast opponent.