Kawasaki Ki-61 Hien (Tony)

The Kawasaki Ki-61 Hien (三式戦闘機「飛燕」, "Flying Swallow"), codenamed "Tony" by the Allies, was first flown in December 1941 and was formally adopted in 1943. It was the only mass-produced Japanese fighter powered by a liquid-cooled inline engine – the Kawasaki Ha-40, a licensed copy of the Daimler-Benz DB 601A.

The Hien's sleek, inline-engine profile was so unlike typical Japanese radial-engine fighters that the Allies initially believed it was an Italian or German-made aircraft. It carried a mix of 12.7mm machine guns and 20mm cannons (the latter being German MG 151/20 in early models, later replaced by Ho-5). Maximum speed was 580 km/h.

The Ki-61 served extensively in New Guinea, the Philippines, and the Home Islands defense. It was one of the main interceptors used against B-29 bombers, although its performance fell off at the bombers' altitude. Total production was roughly 3,150 aircraft (sources give between 3,148 and 3,162). Engine reliability problems plagued the type throughout its career; eventually, airframes without serviceable engines were refitted with the Mitsubishi Ha-112 radial engine, creating the Ki-100, which proved to be an excellent fighter.