‘O-I’ was the chosen designation for a series of proposed super-heavy tanks to be used by the Japanese in the Pacific War. With lessons learnt from the Battles of Khalkhin Gol against the Soviet Union, the Japanese made attempts to improve their tank designs. The Type 95 Ha-Go light tank and the Type 97 Chi-Ha medium tank had proved to be no match against Soviet armour, so a much larger and more powerful tank was urgently required.
In early 1940, Hideo Iwakuro (a colonel with the Army Ministry of Japan) orderedthe development of a new weapon; a super-heavy tank with a proposed weight of at least 100 tons, and equipped with a Type 92 105mm cannon as its main armament. Work was undertaken by Mitsubishi Heavy Industries Tokyo Machinery Division, with the new 120-ton vehicle being designated ‘Mi-To’ (Mitsubishi-Tokyo). The name was changed to ‘O-I’ ( ). ‘ ’ ‘is an abbreviation of ‘ ’ (transl. ‘large’) and ‘ ’ in Japanese army nomenclature, refers to model number 1, from the ancient iroha alphabet.
Supplementing the mainarmament, two smaller turrets would be fitted at the front; one mounting a 47mm cannon, the other a 7.7 machine-gun. To the rear would be two smaller turrets, each mounting a 7.7mm machine-gun. To fulfil its intended role, the tank would have been protected by armour up to a maximum of 200mm, and would have been powered by a pair of V-12 petrol engines (des