Although test pilots were enthusiastic about its self-sealing fuel tanks, upgraded armament, and good dive performance, the wing loading of 146.3 kg/m 2 (30 lb/ft 2) 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/m 2 (19 lb/ft 2) (and even that was considered borderline compared to the earlier Ki-27). 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. A single weapon (initially a 7.7 mm/0.303 in Type 89 machine gun) was able to be carried in a weapons bay located behind the main spar. Each wing had a partially self-sealing 190 L (50 US gal) fuel tank behind the main spar, just outboard of the fuselage. The undercarriage track was relatively wide at 4 m (13 ft 1.5 in). The rear spar carried the split flaps and long, narrow- chord ailerons, while the front spar incorporated the undercarriage pivot points. The evenly-tapered wings had an aspect ratio of 7.2 with a gross area of 20 m 2 (215.28 ft 2) and featured three spars a Warren truss main spar and two auxiliary spars. 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. The windshield was armored and there was a 13 mm (.51 in) armor plate behind the pilot. A self-sealing fuel tank with a capacity of 165 L (44 US gal) was located behind the pilot's seat. The ammunition capacity was limited, having only around 250 rounds for each weapon. The Ho-103 was a light weapon for its caliber (around 23 kg/51 lb) and fired a light shell, but this was compensated for by its rapid rate of fire.
The breeches partly projected into the cockpit, above the instrument panel. Behind the engine bulkhead were the ammunition boxes feeding a pair of synchronized 12.7 mm (.50 in) Ho-103 machine guns which were set in a "staggered" configuration (the port weapon slightly further forward than that to starboard) in a bay just above and behind the engine. A tapered, rectangular supercharger air intake was located on the port-side cowling. For servicing or replacement, only the top and bottom cowling panels could be removed. 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 all-metal, semi-monocoque fuselage was basically oval in cross-section, changing to a tapered, semi-triangular oval behind the cockpit canopy, with a maximum depth of 1.35 m (4 ft 5 in). 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. Priority was given to the Ki-60, which first flew in April 1941, while design work on the Ki-61 did not begin until December 1940. īoth 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. The Ki-60 was to be a heavily armed specialised interceptor, with a high wing loading the Ki-61 was to be a more lightly loaded and armed general-purpose fighter, intended to be used mainly in an offensive, air superiority role at low to medium altitudes. Production aircraft would use a Kawasaki licensed DB 601, known as the Ha-40, which was to be manufactured at its Akashi plant. The Ki-61 was designed by Takeo Doi and his deputy Shin Owada in response to a late 1939 tender by the Koku Hombu for two fighters, each to be built around the Daimler-Benz DB 601Aa.