|Outline of the O-I from record books|
Development of the super-heavy project was spearheaded by Colonel Hideo Iwakuro, the eventual head of the Ministry of War of Japan (陸軍省 Rikugun-shō). Iwakuro opposed Japan’s advances towards the Soviet Union in 1939, and with the Japanese defeat, he decided to initiate a project to construct a heavily armored tank capable of withstanding large-caliber field cannons. Iwakuro assigned Colonel Murata of the 4th Technical Research Group to design and construct the super heavy tank in 1939. Colonel Murata noted Iwakuro’s words as described;
The 4th Technical Research Group began designing the super-heavy vehicle throughout 1940, attempting to meet Colonel Iwakuro’s vague instructions on the ultimate goal of the project. By March 1941, the research group had finished initial tank design and was ready to begin construction. The following month, a group of pre-selected engineers were chosen to partake in the building of the super-heavy tank. One recorded engineer was Shigeo Otaka, who stated they were sent to the 4th Technical Research Group’s previous headquarters in Tokyo. There, they were guided through a barracks containing multiple small fitting rooms, where they were to conduct meetings and reports on the progress of construction of the super-heavy vehicle. Towards the end of the barracks facility was a fully-enclosed room devoid of windows, with soundproofed walls to prevent external personnel from overhearing discussions related to the project. Each officer present possessed a portion of the project’s blueprint, which, when assembled, projected the full design of the tank, labeled "Mi-To". The name originated from a collection of the Mitsubishi industry and the city, Tokyo; given to the vehicle to uphold secrecy of the tank’s project.
|The "Mi-To" shown when the finalized design was completed.|
On April 14th 1941, the engineers began the construction of the Mi-To under secretive means. This entailed privately-made mechanical parts and equipment being shipped to the construction zone. Colonel Murata’s original concept was to complete the super-heavy tank three months after the initiation of Mi-To’s construction. This, ultimately, did not come into fruition; as technical issues on the project began to arise. Due to the limitation on material consumption by the government, the amount of parts that could be secretly shipped-in began to dwindle. By the first month of construction, essential construction resources had been depleted and the issues with the vehicle’s cooling system further caused delays. The construction of the Mi-To was postponed until January 1942, a delay of nine months.
|Top - Down assembly of the Mi-To.|
After the Mi-To’s construction was resumed, the hull was completed on February 8th 1942. The tank had reached near-completion and was being prepared for mobility testing. Mitsubishi built the four turrets for the tank in May of the same year. Initial assembly of the tank’s armament took place soon after the turret’s superstructures were completed. However; the project once again did not have the necessary resources needed for the few remaining parts required for the final assessment. Due to this, the primary turret was removed as it lacked a 35-millimeter-thick roof plate, which had not yet arrived. Thus, the project was put on standby, until further development could continue. The total weight of the vehicle at the time was 96 tons, due to the lack of remaining structural plates and absent 75mm bolted-on armor.
The date on which the construction of the tank resumed is unknown, although active testing of the tank was scheduled for late 1943. The tank was unveiled to the Imperial Japanese Army’s highest command in 1943, and received a name change to O-I. This followed Japanese naming convention (O translating to Heavy, I for First, making it "First Heavy") that was standard. In his place was Lieutenant Colonel Nakano, Murata's assistant and colleague. Tomio Hara, head of the Sagamia Army Arsenal, was also present. Following the demonstration, senior officials within the IJA requested that field trials begin in August of the same year. The tank was disassembled at 2:00 AM one night in June of 1943 and sent to the Sagami Army Arsenal in Sagamihara, 51 kilometers from Tokyo. The vehicle arrived at the depot in June, and was reassembled and tested on the 1st of August.
|Driver's periscope located center front hull.|
On the day of the trials, the O-I performed satisfactorily until the second hour of the tests. While maneuvering on off-road terrain, the tank sank into the ground by up to a meter; attempts at traversing the hull to extricate the vehicle proved fruitless, resulting in further sinking due to the vehicle’s suspension coils compressing. The tank was eventually towed out, and further testing was continued on concrete. However, the earlier damage to the suspension resulted in vehicle’s movement damaging the concrete, which in turn, further damaged the suspension bogies to the point that further testing could not continue. The trials were postponed, and later canceled the following day.
Nevertheless, the trials conducted at the testing field were considered to be a success, and the vehicle was deemed ready for use in spite of its flaws. The engineers began disassembly of the tank on the 3rd of August due to resources being limited and the inability to maintain the tank in the field. Disassembly of the tank was completed on August 8th. Two days later, the engineers noted in a log that they were to inspect the parts and conduct research to fix the issues the O-I would face.
|Surviving chain link of the prototype.|