Ki-61 Hien "Tony"

The Kawasaki Ki-61 Hien (飛燕, Swallow), designated Army Type 3 Fighter (三式戦闘機), was the only Japanese production fighter with an in-line liquid-cooled engine during WWII. Allied forces codenamed it "Tony" — originally thinking it was Italian due to its un-Japanese appearance.

Development

In February 1940, the Army ordered Kawasaki to develop a fighter around the Ha-40 engine — a Kawasaki-licensed version of the German Daimler-Benz DB 601 (the same engine in the Bf 109E and Macchi MC.202). Detailed design began December 1940, and the prototype flew in December 1941.

Test results were spectacular: the Ki-61 achieved 591 km/h in initial trials, decisively faster than any in-service Japanese fighter. The Army immediately ordered production, and the aircraft was officially adopted in 1943 (June or October, depending on the source) as the Type 3 Fighter.

Combat Service

The Ki-61 first saw combat over New Guinea in 1943, where it shocked Allied pilots who had expected another Zero-type fighter. Instead, the Hien dove well, absorbed punishment better than most Japanese aircraft, and had effective firepower. However, the Ha-40 engine proved troublesome in the field — prone to overheating and requiring skilled maintenance.

Over Japan in 1944–1945, the Ki-61 became the primary interceptor against B-29s. The 244th Air Group (飛行第244戦隊), defending the Tokyo area from Chofu Airfield, became legendary for aggressive head-on attacks against the Superfortresses.

When engine production collapsed — the Akashi engine plant was destroyed by bombing on January 19, 1945 — ~275 engineless Ki-61 airframes were fitted with the Mitsubishi Ha-112 radial engine, creating the Ki-100 — arguably Japan's finest home-defense fighter.

Specifications (Ki-61-I-KAIc)

• Engine: Kawasaki Ha-40 liquid-cooled V-12, 1,175 hp
• Max speed: 580 km/h at 5,000m
• Empty weight: 2,630 kg
• Armament: 2× 20mm Ho-5 cannon + 2× 12.7mm Ho-103

Surviving Aircraft

The sole Ki-61-II-Kai (serial 6117) is preserved at the Gifu-Kakamigahara Air and Space Museum. Owned by the Japan Aeronautic Association, it has been recognized by the association as an Important Aviation Heritage (重要航空遺産).