The design was a twin-engined, low-winged monoplane with a retractable underwing carriage. It had a small diameter oval-shaped fuselage with room for a crew of two with the pilot and the observer placed in individual cockpits separated by a large fuel tank. Extra fuel tanks were placed in the thin wings both inboard and outboard of the engines, giving a total fuel capacity of 1.490 L (328 Imperial Gallons). It was powered by two Mitsubishi Ha-26 radial engines which, in the first variant, they were rated at 863 hp each and were housed in close fitting cowlings developed by the Aeronautical Research Institute of the Imperial Tokyo University with the aim of reducing drag and improving pilot's field of view.
The first prototype flew in November 1939 from Mitsubishi's factory in Kakamigahara, Gifu, North of Nagoya. After being tested, it proved to be seriously underpowered and was slower than required by the specification as it only reached 540 km/h (336 mph) instead of the specified 600 km/h (373 mph). Regarding other aspects, testings were successful. Considering that the aircraft was still faster than the IJAAF's main fighter, the Nakajima Ki-43 and faster too than the Imperial Japanese Navy Air Service's (IJNAS) Mitsubishi A6M2 Zero (newly introduced into service back then) an order for an initial production was placed by the IJAAF as the Army Type 100 Command Reconnaissance Plane Model 1 (Ki-41-I).
In order to solve performance problems, Mitsubishi replaced the engines with Ha-102 radial ones which were able to yield up to 1.065 hp of power. These engines were basically Ha-26 fitted with two-speed supercharger. On this improved design, fuel capacity was also increased and empty weight was also reduced. It was in flying condition in March 1941 and was commonly known as Ki-46-II. This variant met the speed requirements of the original specification and was therefore ordered in a full-scale production with deliveries starting in July.
At first the Ki-46 was virtually inmune to any fighter interception, however it was soon realised by the IJAAF that improved Allied fighters such as the Supermarine Spitfire the Lockheed P-38 Lightning could contest this immunity, so in July 1942 they instructed Mitsubishi to produce a further improved version as the Ki-46-III. This variant was powered by two fuel-injected Ha-112 radial engines which were able of yielding 1.479 hp of power, it had a redesigned nose with a fuel tank ahead of the pilot and a new canopy, smoothly integrated into the extreme nose of the aircraft eliminating therefore the typical 'step' of the earlier versions. The single defensive machine gun present in earlier versions was also deleted. This new variant flew for the first time in December 1942 and showed significant higher speed 630 km/h (391 mph) at 6.000 m (19.700 ft) high. Thanks to its performance, it proved to be superior to designs made by other rival companies, such as the Tachikawa Ki-70 which didn't enter production.
Trying to achieve even more altitude performance, two prototypes were fitted with exhaust driven turbosupercharged Ha-112-II-Ru engines. These flew for the first time in February 1944.
During its production period ranging from 1941 to 1945, a total of 1.742 exemplars (other sources claim it was 1744) were manufactured. Thirty-four Ki-46-I, 1.093 K-46-II, 613 Ki-46-III and 4 Ki-46-IV all of them at Mitsubishi Heavy Industries, in Nagoya, Japan.
Sources:
1. https://en.wikipedia.org/wiki/Mitsubishi_Ki-46
2. https://www.valka.cz/Micubisi-Ki-46-Sin-Sintei-Dinah-t28814 (translated when needed)
3. Bunrin Do - Famous Airplanes of the World 38 - Mitsubishi Ki-46 (translated)
3a: Translated captions can be found at: http://www.arawasi.jp/Dinahtrans.html
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