[4][5] The Ki-61 was designed by Takeo Doi and his deputy Shin Owada in response to a late 1939 tender by the Koku Hombu[N 1] for two fighters, each to be built around the Daimler-Benz DB 601Aa.

[N 3] Both single-seat, single-engine fighters used the same basic construction: all-metal alloys with semi-monocoque fuselages and three-spar wings, with alloy-framed, fabric-covered ailerons, elevators and rudders.

Although the Ki-61 was broadly similar to the Ki-60, it featured several refinements exploiting lessons learned from the disappointing flight characteristics of the earlier design.

An unusual feature of the Ki-61 was that the engine bearers were constructed as an integral part of the forward fuselage, with the cowling side panels being fixed.

The radiator and oil cooler for the liquid-cooled engine were in a ventral location below the fuselage and wing trailing edge, covered by a rectangular section fairing with a large, adjustable exit flap.

[6] The first prototype of the San-shiki-Sentohki ichi gata ("Type 3 Fighter, Model 1", the official IJAAF designation) first flew in December 1941 at Kagamigahara Airfield.

[7] Although test pilots were enthusiastic about its self-sealing fuel tanks, upgraded armament, and good dive performance, the wing loading of 146.3 kg/m2 (30 lb/ft2) at an all-up weight of 2,950 kg (6,500 lb) was viewed with scepticism by many of the senior officers of the Koku Hombu, who still believed in the light, highly manoeuvrable, lightly armed fighter epitomised by the then new Nakajima Ki-43-I-Hei which had a wing loading of 92.6 kg/m2 (19 lb/ft2) (and even that was considered borderline compared to the earlier Ki-27).

Moreover, the inline Ha-40 engine proved to be an unreliable powerplant,[12][13] exacerbated in the South Pacific campaign, where high temperatures caused problems with the fuel pumps.

Reaching these standards proved difficult for Japanese manufacturers, an issue further complicated by the variable quality of materials, fuel, and the lubricants needed to run a sensitive, high-performance engine.

While the Ha-112 solved the problems encountered with the Ha-140, the new engine still had a major weakness: a lack of power at altitude, which diminished its ability to intercept high-flying B-29 Superfortresses relative to the Ki-61-II.

[citation needed] The Ki-61 looked so different from the usual radial-engined Japanese fighters that the Allies at first believed it to be of German or Italian origin, possibly a license-built Messerschmitt Bf 109.

Both units were sent into a difficult theatre where jungles and adverse weather conditions, coupled with a lack of spares, quickly undermined the efficiency of both men and machines.

General George Kenney, the Allied air forces commander in the Southwest Pacific, found his Curtiss P-40s completely outclassed, and begged for more Lockheed P-38 Lightnings to counter the threat of the new enemy fighter.

[13] The Ki-61 was also utilised in Southeast Asia, Okinawa, China and as an interceptor during US bombing raids over the Japanese home islands, including against Boeing B-29 Superfortresses.

[12] Owing to the additional weight, the Ki-61's performance and agility suffered when its armament was increased, but it still remained capable with a 580 km/h (313 kn) maximum speed.

The cannon armament was needed to counter the Allied bombers, which proved to be difficult to shoot down with only 12.7 mm (.50 in) machine guns.

The aircraft was largely trouble-free in service except for the liquid-cooled engine which tended to overheat when idling on the ground and suffered from oil circulation and bearing problems.

[25] The tactic of using aircraft to ram American Boeing B-29 Superfortresses was first recorded in late August 1944, when B-29s from Chinese airfields attempted to bomb the steel factories at Yawata.

Despite their successful attacks, these pilots gained no reprieve and were obliged to continue these deadly and dangerous ramming tactics until they were killed or wounded so badly that they could no longer fly.

[32] Some other Ki-61 pilots also achieved renown, among them Major Teruhiko Kobayashi of the 244th Sentai, who was credited by some with a dozen victories mostly due to conventional attacks against B-29s.

By 1944, with the depredations of Allied attacks on supply lines and airfields, as well as the loss of pilots and aircraft through combat attrition and accidents, few sentai were able to operate at full strength.