In 1937, the Japanese firm of Mitsubishi Heavy Industries Company delivered a new reconnaissance aircraft, the "Ki-15", to the Japanese Imperial Army. The Ki-15 was a clean, single-seat, single-engine monoplane with fixed landing gear and excellent range, and though it appeared useful enough for the moment, the Technical Branch of the Imperial Army Air Headquarters (Koku Hombu) knew that other nations were developing fighters fast enough to overtake and destroy it.

Within two months of the first service delivery of the Ki-15, the Koku Hombu began work on the specifications for its successor, an improved reconnaissance aircraft that was to discreetly overfly lands belonging to Japan's potential adversaries. The requirements that were defined by the Koku Hombu's Major Yuzo Fujita and his staff were aggressive, dictating an aircraft with a top speed of 600 KPH (373 MPH) at 4,000 meters (13,100 feet). This was much faster than any Japanese aircraft that had flown to that time.

The new aircraft also was to have an endurance of six hours at 400 KPH (250 MPH) at an altitude of 4,000 to 6,000 meters (13,100 to 19,700 feet). The Army air staff knew that building an aircraft with such capabilities would not be easy, and so gave industry designers a generally free hand in designing whatever they thought could do the job. The aircraft could have one or two engines, using air-cooled radials in the 560 to 710 kW (750 to 950 HP) class, such as the Nakajima Ha-20-Otsu, Nakajima Ha-25, or Mitsubishi Ha-26.

* Mitsubishi was already designing a fast twin-engined reconnaissance aircraft designated the "Ki-40", a variant of the company's proposed Ki-39 twin-engine fighter. Because of their established work, Mitsubishi was awarded the contract for the new reconnaissance aircraft by the Koku Hombu on 12 December 1937.

Although the Ki-40 hadn't been flown by that time, the Mitsubishi design team, led by Tomio Kubo and Joji Hattori, realized very quickly that there was no way it could be fast enough to meet the Koku Hombu specification. They junked the Ki-40 design, retaining only some of its features in a new, much more streamlined and elegant twin-engine aircraft with low-mounted thin wings.

The new design was given the designation "Ki-46". It featured a forward crew compartment for the pilot and a separate crew compartment facing the rear for the radio operator, with the two compartments separated by a bay containing cameras and a large fuel tank with a capacity of 1,660 litres (440 US gallons). The unusual crew accommodations were dictated by the need to put the big fuel tank at the centre of gravity. It had "tail-dragger" landing gear, with a retractable tailwheel and the main gear retracting back into the engine nacelles.

The twin powerplants were Mitsubishi Ha-26-Ko 14-cylinder air-cooled radial engines with single-speed superchargers, with each engine providing 746 kW (900 HP). The Nakajima Ha-25 radial was lighter and more powerful, but Mitsubishi preferred to supply their own engines.

The Mitsubishi design team had worked with the Aeronautical Research Institute of the University of Tokyo to perform wind-tunnel tests for streamlining the aircraft and in particular to optimize the engine fit, coming up with aerodynamic, close-fitting cowlings and large spinners fitted over the three-bladed constant-speed variable-pitch propellers. The tight cowlings also improved the pilot's field of view.

* Despite such efforts, when flight trials began in 1939 with Major Fujita at the controls, the Ki-46 did not meet the speed requirements requested by the Koku Hombu, attaining only 540 KPH (335 MPH) at 4,000 meters (13,100 feet). The Koku Hombu still found it an excellent aircraft in all other regards, and so the Ki-46 was accepted for production as the "Army Type 100 Command Reconnaissance Aircraft Model 1 (Ki-46-I)".

The Koku Hombu did specify that Mitsubishi was to immediately begin work on a faster version, the "Ki-46-II". Since the Ki-46-I's relatively low performance meant that it was in principle vulnerable to interception, the back-seat position was fitted with a single moveable Type 89 7.7 millimetre (0.303 calibre) machine gun to provide a minimal self-defence capability.

At the time, Mitsubishi was heavily committed to building other aircraft, and production of the Ki-46-I was slow. Manufacturing problems were aggravated by the fact that the Ki-46 had been designed for high performance, at the expense of ease of manufacture and maintenance. A few were delivered for Army evaluation during the spring of 1940, and in a short time a number of them were provided to the Shimoshizu Rikugun Hikogakuko (Shimoshizu Army Flying School) for crew training.

By the spring of 1941, the Army had at least 386 Ki-46s on order, but they were still only being delivered at the rate of four a month. Mitsubishi was ordered to stop production of some older aircraft and shift resources to building the Ki-46, and by November 1941 deliveries reached ten aircraft a month. Monthly production would continue to increase, to a peak of 75 aircraft delivered in March 1944.

Various teething problems and weaknesses were uncovered as the Ki-46 was put into the hands of operational pilots. Trials in Formosa revealed that engine vapour lock was a considerable nuisance under hot and humid conditions. The problem was fixed with a small change in the position of fuel lines around the engine, and a change to higher octane fuel.

The main landing gear also proved to be too weak, often collapsing on hard landings, which were fairly common due to the Ki-46's high wing loading. Although some minor fixes were implemented, the Ki-46 suffered from weak landing gear all through its life. The Ki-46 also proved un-manoeuvrable and had a sluggish rate of climb, partly due to a tendency for the oil to overheat. However, the Ki-46 was not intended for air combat, and these limitations were acceptable.

Some Ki-46-IIs were fitted with a radio compass for long range navigation, with such machines identifiable by a teardrop-shaped directional antenna on a short pylon between the front and back cockpits. Later in the war, a number of Ki-46-IIs were modified into three-seat radio navigation trainers through the installation of a stepped-up secondary cockpit behind the pilot's position. This variant was designated the "Ki-46-II Kai", where "Kai" was short for "kaizen (improvement)".

* The new Ki-46 reconnaissance units engaged in probes of China and other areas that the Japanese military hoped to seize in their plans for all-out war in the Pacific. In October 1941, Ki-46s flew from Cambodia to survey possible amphibious landing sites in Malaya.

When the war finally broke out in December 1941, the Japanese offensive rolled over Western colonial possessions in the Far East like a tidal wave. Within months, the Japanese had seized Hong Kong, Malaya, Singapore, Burma, the Philippines, and the Dutch East Indies. They established bases over the western Pacific to protect their new empire.

The Ki-46 was a very useful tool in their military operations. Operating from bases in Timor in what had been the Dutch East Indies, the Ki-46 flew far over northern Australia, and operating from bases in Burma the Ki-46 was able to observe British naval activities in Ceylon, 1,600 kilometres (1,000 miles) distant.

The Japanese Imperial Army had shown unusual foresight in obtaining a specialized high-speed reconnaissance aircraft, and the Imperial Japanese Navy, which wasn't usually inclined to agree with the Imperial Japanese Army on anything, recognized the merit of the Ki-46 to the extent of obtaining a small number of the aircraft from the Army.

In the early stages of the war in the Pacific, the Allies were reduced to improvising vulnerable bombers and transports to the reconnaissance role. The Ki-46, in contrast, could operate with impunity, since it was faster than any fighters the Allies had in the region. Even when improved Allied fighters became available, the Ki-46 proved difficult to catch.

The Allies quickly recognized the Ki-46 as an impressive aircraft. In late 1942, they gave it the codename "Dinah", and intelligence personnel described it as the "Dinah with the nice linah!" The Germans were interested enough the Ki-46 to consider obtaining a manufacturing license for it, but nothing came of the exercise.

Mitsubishi KI-46-II:
 
specs                             

wingspan  48 feet 2 inches
wing area  344.5 sq_feet
length  36 feet 1 inch
height  12 feet 8 inches

empty weight  7,190 pounds
max loaded weight  12,790 pounds

maximum speed   375 MPH / 325 KT
service ceiling  35,170 feet
range  1,540 MI / 1.340 NMI