The Ki-20; Mitsubishi’s Monstrous White Elephant

July 29, 2023

Japan in the 1920s and -30’s was a nation sliding inevitably into militarism. The root causes of this went back decades but by this point Japan had firmly established itself as an imperial power, having conquered Korea, Formosa (now known as Taiwan) and dominating large swathes of northern China.

The Japanese military, who were playing a rapidly expanding role in forging policy, were keen that the Empire would continue to expand. And air power was recognized as being an important part of this.

Japanese manufacturers had spent the 1920s largely building foreign aircraft under licence, followed by recruiting designers from abroad to assist in creating indigenous aircraft designs. Naturally, as an island nation and with aspirations that stretched across the Pacific and the vast distances of China and the Siberian steppe, the Japanese were thinking about the practicalities of how to maximise the effectiveness of their air strength.

For the Imperial Navy, that essentially meant aircraft carriers, though they also explored such intriguing ideas as aircraft-carrying submarines. But the Japanese Army, who technically controlled land-based aviation, wanted to have their own long-range air power. However, in the late-1920s an aircraft that could carry a worthwhile bombload over a reasonable distance would have to be truly massive.

Meet the Mitsubishi Ki-20.

To meet the Imperial Army’s requirements Mitsubishi turned to one of the foremost builders of long-range aircraft of the day – Junkers. That company was working on a new airliner, the G.38, a huge machine that carried its passengers in the aircraft’s thick wings. They also apparently conducted a design study on whether a militarized version was feasible.

Seeing as the German military at the time was highly limited by the constraints of the Treaty of Versailles and in fact didn’t have an air force, I’m not sure what Junker’s were hoping to achieve with this, if indeed it is actually true. But the Japanese certainly thought that the massive machine would make an excellent basis for a bomber and, even before the G.38 first flew, had approached Junkers in 1928 with a view to having them jointly design and build a military variant for Japanese use. The agreement would entail the purchase of drawings and parts for production of the new bomber, which was type designated as the Type 92 Heavy Bomber by the Japanese army.

The first two aircraft were built utilizing German made components and the Ki-20 first flew in 1932. Japanese construction then took place, but at a very limited scale, as only four more would be completed by 1935 when production ceased.

Of all metal construction the aircraft was certainly prodigious. The wingspan was 44m (144’ 4”) and the fuselage 23.2m (76’ 1”) in length.

Compare that to the Martin B-10, an aircraft that was being built and entering service at the same time and which was the most advanced bomber flying, which had a wingspan of 21.49m (70’ 6”) and a length of 13.64m (44’ 9”).

Of course, the Ki-20 was classed as a heavy bomber in comparison to the B-10, but its defensive armament and offensive payload was on another level. While the B-10 had three .30-calibre machines guns and carried a bombload of 2,260lb (1,025kg), the Ki-20 had six defensive gun mounting positions, made up of twin .303-calibre machine guns in the nose, two more twin guns in positions located above the wings, two single-machine gun in cupolas under the wings and, to top it all off, a 20mm Oerlikon cannon in a dorsal position.

This was a staggering amount of defensive armament for an aircraft of the time, which wouldn’t be superseded until well into the Second World War.

And in keeping with its massive proportions the Ki-20 was capable of carrying a maximum bombload of around 11,000lb (c.5,000kg). The crew was similarly large, with ten men being needed to operate the Ki-20.

But of course, there were downsides to all this. For starters, engines available in the early-1930s were rather limited. Initially the Ki-20s were fitted with four Junkers L88s, but these proved problematic and were replaced by Junkers 204 diesel engines that produced 750hp each.

This was enough to get the monsters airborne just about, but performance was very limited, with the Ki-20s having a top recorded speed of just 120mph (c.200km/h) and a cruising speed of 78mph (125km/h).

Needless to say, though the Ki-20s appeared impressive and were heavily armed, the Japanese recognized that their deployment in a military situation would have to be extremely carefully considered. So, the Japanese army created an elite squadron for the aircraft and considered what they were going to use these behemoths for.

Because of their heavy payload it was thought that they may be of use for attacking strategic targets in Siberia, or else for potential use against the heavily protected coastal fortifications that the Americans had built to protect their strategic harbor at Manila on the Philippines. Then with Japan’s full invasion of China in 1937 thought was apparently given to using them against Chinese cities, where their heavy payload would be useful in making them a terror weapon.

But the Ki-20s appear to have been stymied by two factors.

Even in the lighter air threat environment of China, where the Japanese generally enjoyed air supremacy, the Ki-20s truly lumbering performance meant that they would be absolute sitting ducks to any Chinese fighter that managed to intercept them. Plus, though Chinese air defences were equally sporadic in their appearance, there could be no doubt that a Ki-20 would provide excellent target practice for any gun crew they happened across.

Additionally, the Ki-20 proved to be one of those weapons where the perceived need to keep them secret precluded them from being used for anything at all. The Japanese Army seem to have been determined to keep the existence of the Ki-20s from any other nations knowledge – no doubt waiting for the perfect moment to unleash their fearsome new heavy bomber upon an unsuspecting enemy.

Except that this time never came and the Ki-20s were obsolete within five years.

This seems to have been recognized finally by the Japanese in 1940, when one flew for the first time in public at an air parade over Tokyo.

Little seems to be known about their subsequent use in World War Two, though they appear to have been used as transports, which to be fair is all they would be really suitable for by that point. This seems to have ended in 1943 when their use as a source of critical raw materials seems to have superseded their value in service and five of the six were scrapped.

The final example was apparently placed in the Tokorozawa Aviation Museum on display. But this too disappeared in the post-war chaos, assumably also broken up for scrap.

And that concludes the tale of the Mitsubishi Ki-20 heavy bomber. A complete white elephant of an aircraft that was an example of both how ideas can be ahead of their time and the danger that concerns for secrecy can effectively neutralize a weapon system even better than an enemy could.


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