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Type 91 Heavy Tank prototype

In 1928, although the Japanese had not decided on the trial production of the No. 1 tank, it decided to continue to develop a heavy tank to gain an advantage in the anticipated war in China. In the same year, the project was established. In 1931, a prototype was begun and in March 1932, the prototype was completed. According to the custom of the Japanese Imperial Army, this newly developed tank won the name of Type 91 Medium Tank.

The tank had 3 turrets, a main one in the center armed with a 57 mm (2.24 in) cannon, and 2 secondary forward and rear turrets armed with 6.5 mm (0.26 in) machine guns. This tank continued the design of the first tank, and a small machine gun turret is installed in the front of the vehicle body, and one is also set at the rear of the hull. Unlike the trial of the No. 1 tank, a machine gun was added to the left rear of the main turret of the vehicle. These three parts are equipped with the same type of 6.5mm machine gun.

The vehicle armor plate is riveted and the front armor is 20mm. It also had armor 8 – 17 mm thick. However, due to weight control, the weight of the whole vehicle is the same as that of the trial tank No. 1, a total of 18 tons.

This 18-ton vehicle was powered by a 6-cylinder BMW IV inline engine that ran on gasoline. The water-cooled inline six-cylinder petrol engine was likely purchased from Germany. The engine provide 224 horsepower, with a maximum speed of 25km/h. A feature carried over from the Type 87 Chi-I was the rather complicated parallelogram suspension system with two pairs of road wheels per leaf-sprung bogie. There were 17 road wheels per side, giving it a total of 34 road wheels. The parallelogram type of suspension is more commonly known as Swing arm suspension, which is mostly used on motorcycles.

At the beginning the main gun was still used to test the unfinished 57mm short-barrel gun on the No. 1 tank, and then changed to an unknown model of the 70mm diameter 18.2 calibers (presumably it was later installed in the Type 95 tank). The prototype gun that was developed in 1932 was the Type 94 57mm gun. Due to the complicated design of the suspension, the entire plan was stopped after several experiments on the prototype. The Type 91 proved to be an unsuccessful endeavor, and the project was soon canceled. The lessons learned were passed on to the Type 95. In December 1932, the new Type 95 tank plan based on the preceding vehicle was established.