Armoured Sumo Fighters: Japanese Heavy Tanks

By the 1930s, Japan’s influence in the Far East was great. Among the things that assured the country’s successful expansion during this period was the attention paid by the upper echelons of Japanese military command to technical innovations, particularly in the field of military armaments. Of course, the Japanese were also interested in armoured vehicles.

Japan started importing tanks and other armoured vehicles around approximately 1917 and in less than a decade they had developed their own tanks. The tanks developed were medium and light tanks, capable of engaging lightly armoured military units, such as ground forces or island-based garrisons, which lacked the heavy armaments needed to deal with armoured units.

The history of Japanese armoured vehicles still has an underexplored chapter – heavy tanks. Information about the work of the designers on these vehicles is limited and often contradictory. Nonetheless, there is something we can tell you about them.

One thing can be said for sure: Japanese engineers were working on heavy tanks long before World War II. Their initial prototype was named the Type 91 or 2591 and work on its design was started in 1930. When creating their first heavy tank, the engineers used experience gained from the creation of the medium tank, Chi-I. Tomio Hara, one of the more renowned Japanese engineers developing armored vehicles at the time, was a member of the design team that worked on the project.

In the pre-war period, Japanese engineers developed and created two heavy tanks: the Type 91 and the Type 95. Neither of them were mass-produced or actively participated in combat.

The Type 91 weighed only 18 tons, making it a medium tank according to Western European and Soviet classifications. However, the tank was equipped with a 70mm gun, so the vehicle was considered a heavy tank. Influenced by 1930s British tank-school theory, which preferred multi-turreted vehicles over the single turret now common to almost all tanks, the designers equipped the Type 91 with three turrets placed along the hull.

Only one prototype was created. Despite making a good impression, the vehicle was not sent to mass production, with Japanese military command instead demanding a modernisation.

The Type 95, the second and last pre-war heavy tank, came into service in the Japanese army in 1935. When creating the vehicle, the designers focused on increasing the firepower. In addition to the 70mm gun, which had a low muzzle-velocity intended for use against protective structures and personnel, the vehicle was equipped with another 37mm gun capable of penetrating up to 30mm of armour from a distance of 300 metres. This gun made the Type 95 a potentially dangerous opponent for the thinly armoured Soviet T-26 and the BT-7, should they ever meet.

The new tank’s weight was increased to 27 tons. Again, according to Western and Soviet classifications, this vehicle was considered a medium tank but the Japanese thought differently. At that time in the Far East, the Type 95 had no rivals in terms of armour or weaponry, so it made sense to classify the vehicle as a heavy tank. At the same time, the military had contradictory opinions – they liked the Type 95’s combat characteristics but its lower speed, due to the vehicle’s weight, disappointed them. It was decided to produce a limited experimental series and the order was sent to the Osaka Army Arsenal. The number of vehicles produced is unknown but, according to existing sources, the production run was less than ten tanks. The Type 95 appears to have been used for training units within Japan's territory. However, it is known that in 1938 several tanks were sent to China and used in the final stage of the Second Sino-Japanese war.

Super-Heavy Ghost

One of the attempts at appearance reconstruction of the heavy tank O-I.

If the O-I had been built, it would have represented the pinnacle of Japan’s tank-building prowess. Little is known about this super-heavy tank and it is often contradictory. Only three things are known for sure:

Fact #1: Work on the super-heavy tank began after Japan was defeated in the Battles of Khalkhin Gol in 1939.

Fact #2: The name O-I was given to the vehicle by the Japanese Army. The Mitsubishi-Heavy Industries (MHI) referred to the vehicle as the “Mi-To,” as it was created in the MHI Tokyo Machinery Division. Because its development was interrupted by the war, the O-I and the Mi-To are often mistaken as two different projects but recent findings have revealed them to be the same thing.

Fact #3: All of Japan’s super-heavy tank projects were multi-turreted. 

The development of the O-I was done under extreme secrecy and it was designed in a small isolated room inside a barracks. Shigeo Otaka, an engineer who participated in its development, revealed that the room was separated from the rest of the factory by a double door, intended to minimise the risk of someone accidentally wandering inside. Different departments within Mitsubishi worked on different vehicle components without actually being aware of what they were designing. It was the designers working away in the secret room who had authorised access to them all, as it was their job to put the technical puzzle together.

There were a great many technical challenges to be overcome before completing the vehicle. Gradually, the vehicle became refined. As the project progressed, the vehicle became larger and heavier; in its smallest form the O-I is said to have been about 100 tons, and in its final form around 150 tons.

Why would Japan need such a monster? Initially, when no invasion threatened Japan, these tanks were planned as assault vehicles. In 1944, Japan’s defensive perimeter, based around several islands in the Pacific Ocean, was breached. In April 1945, American troops captured Iwo Jima. It quickly became clear that an invasion of Japan was inevitable. Now the super-heavy tank was planned as a mobile fire unit, intended for coastal defense.

Initially, a 150mm howitzer was chosen as its main armament but, in the final stages of the project, naval guns with a calibre of 100-150mm were considered instead. These guns had good range as well as the necessary firepower to deal heavy damage to landing vessels, as well as any covering ships. At the same time, the armour of the super-heavy tanks would provide safe cover from explosions and shrapnel. These 'mobile pillboxes' could be destroyed only with a direct hit, while ships could not accurately fire at such a small target. 

However, the Japanese engineers stopped working on the O-I before the end of the war. The hull was completed, but there were problems with the running gear and the vehicle did not pass field trials. If completed, several variations and upgrades were said to be proposed but further development on this vehicle remains a mystery.

Watch out for these tanks in 9.10!