In 1940 Kawasaki obtained a licence for the Daimler-Benz DB 601, the inverted V-12 that powered the Messerschmitt Bf 109 and Bf 110. A design team returned from Germany with drawings and sample engines, and the first Kawasaki Ha-40 was completed in 1941. On paper it was excellent: lighter than the German original and rated around 1,175 hp. In practice the Ha-40 was where Japanese production limits showed most cruelly — it ran hot, its bearings and tolerances were unforgiving, and reliability in the field was poor. The uprated Ha-140 of the Ki-61-II pushed power toward 1,500 hp but made the problems worse. The engine that gave the Hien its distinctive lines was also the reason so many never flew.

By 1944 the Hien had become one of the Imperial Japanese Army’s more capable interceptors against the B-29 Superfortress. The improved Ki-61-II could reach the bombers’ high cruising altitude where lighter, radial fighters ran out of breath, and units such as the 244th Sentai defending the Tokyo area became closely associated with the type. As raids intensified and conventional interceptions proved costly, some Hien pilots resorted to deliberate ramming attacks (taiatari) — stripping their aircraft of guns and armour to gain altitude and colliding with the bombers. It was a measure of desperation, and of how far the defenders were prepared to go.

The first production Ki-61-I (from 1942–43) used the roughly 1,175 hp Ha-40 and went through sub-variants with heavier armament, including licensed 20 mm Mauser cannon and later the Japanese Ho-5. The Ki-61-II introduced the more powerful but troublesome Ha-140 and a redesigned wing, aiming at the altitude needed to fight the B-29. Engine supply collapsed: of several hundred Ki-61-II airframes, only a fraction received engines before the Akashi factory was bombed in January 1945. The remaining airframes were completed with a Mitsubishi radial as the Ki-100 — the same basic aircraft, transformed into one of the war’s best-handling Japanese fighters.

The Ki-61 was a wartime Japanese state product, built under a command economy and destroyed or scrapped almost in its entirety by 1945. No credible flyaway unit price or cost-per-flight-hour figure survives in open sources, and any modern dollar figure attached to the type is guesswork. What is documented is the human and industrial cost: an engine so hard to build that finished airframes sat waiting, and a programme of roughly 2,750 aircraft whose potential was throttled by supply, not design.