O-I Superheavy Tank: A Complete History

The Battle of Khalkhin Gol had been a devastating defeat for the Empire of Japan in 1939. The Kwantung Army suffered total strategic defeat at the hands of the Soviet Union surrounding the village of Nomonhon. This had been Japan’s first significant deployment of armour against a modern army. Tank development in Imperial Japan was still in its nascent stages by the time the border conflict at Nomonhon began.

Notebooks belonging to the engineers working on the O-I/Mi-To tank project, and
where majority of the source material from this write-up derived from, including all drawings.

Japan constructed a series of fortifications in Manchuria and central China under the supervision of the Continental Fortification Research Committee, a department under the Imperial Army. This committee oversaw all major defenses in the region against the Soviet Union and Chinese warlords. Shortly after the Nomonhon Incident, the committee submitted an idea to construct special purpose tanks that were capable of breaking through heavily defended Soviet lines and force the Soviet armies back out of taken Manchurian lands. Colonel Hideo Iwakuro, an officer in the Military Affairs Bureau Tank Research Team, took an interest in the Continental Fortification Research Committee’s proposal. Iwakuro sought to build super heavy tanks to fit the Committee's plan: tanks capable of withstanding fire from Soviet artillery and anti-tank guns while supporting infantry and medium and light tanks pushed through Soviet defensive lines.

Type 95 Ro-Go. Japans second heavy tank.

Japan built and tested an array of multi turreted heavy tanks from 1925 to 1938 in an attempt to implement a breakthrough vehicle. However, these tanks all had the same underlying issues; their armour was too thin to protect against anti-tank gun and their armaments were inadequate. Due to weak engines, these prototypes lacked sufficient speed and mobility, but forced the chassis to be larger. 




Colonel Iwakuro set forth to construct a tank to fit the Continental Fortification Research Committee’s vague proposal. In the early months of 1940, Colonel Iwakuro along with a team of twenty engineers from the 4th Technical Research Group to design the superheavy tank at their headquarters in the Tokyo area. The team held meetings in a secluded barracks, complete with multiple small fitting rooms, where they conducted meetings and wrote reports on the progress of the construction of the super-heavy vehicle. At the end of the barracks facility was a fully-enclosed room with no windows and soundproofed walls to prevent external personnel from overhearing discussions related to the project.

The superheavy tank project proceeded under the temporary name Mi-To. This came from the fact that Colonel Iwakuro had hired the Mitsubishi Heavy Industries Tokyo Equipment Manufacturing group producing the parts needed for the tank. Construction work would be performed by the Sagami Army Arsenal in secret. Colonel Iwakuro had taken on this project because he was capable of securing confidential funds from the Military Affairs Bureau, as he had been the section director at the time. This permitted Iwakuro to work on the project without the Army’s head officials knowledge.

Initial draft of the Mi-To's interior and suspension.

The Mi-To maintained the core design Japan had used with earlier heavy tank prototypes; a multi turreted chassis. The design version of the Mi-To weighed in excess of 150 tonnes, heavier than any tank of the era. The weight of this project came from the requirement of being impervious to Soviet anti-tank guns and artillery. The engineers working on the project expressed their skepticism on how feasible the tank would have been. However, the team, originally at 20 people, eventually dwindled to just 2 members. Funding of the project became unstable early in development as Colonel Iwakuro was been sent to a seperate division.

As was custom with Japanese heavy tanks, the Mi-To had a multi turreted system with two auxiliary turrets in the front part of the chassis, one primary turret in the center hull of the tank, and a machine gun turret located at the rear of the chassis. The blueprints for the Mi-To show a Type96 150mm Howitzer, and two new Type1 47mm guns in the front auxiliary turrets. Due to the excessive weight of the turrets, manual rotation would not be possible. Therefore, all four turrets were to be equipped with electric traverse drives.

Due to the heavy weight of the armour, the Mi-To required a special assembly to hold all the main armor plates together as the vehicle would have to be disassembled and reassembled  to send it across Manchuria to deployment zones. This frame was held together by 35mm thick mild steel plating pieces. This gave the vehicle a sturdy structure while minimizing the added weight to the vehicle. According to project engineer Shigeo Otaka, the tank was designed with frontal and rear armour 150 millimeters thick. Held together by the 35mm framing network. The side hull was 35mm thick with a 35mm side skirt for roughly 70mm of total side protection. The design of both frontal and rear protection is peculiar, but due to the design requirement being a mobile fortress, it seemed fitting. 

A primary feature of the super heavy tank was the ability to quickly disassemble the tank to transport it. Because of this, the armour plating were able to detach via the bolt-on frame.  The bottom of the vehicle had two plates welded together to support the weight of the chassis on top.

Construction officially began on April 14th, 1941. The Army Technology Division had taken control on the project after Colonel Iwakuro left. The project was now overseen by Colonel Murata, an officer in the Army Technology Division. Murata had not expected the tank to have been so underfunded when he was assigned the position. He planned to finish body construction within the first three months of the start of the project. Unfortunately, resources had already started to dwindle by the time the frame was halfway through construction. Mitsubishi Heavy Industries had been contracted to build the armour plating for the vehicle. These resources were sent to garage the Mi-To began construction at. After a single month of building, the  Army Technology Division had completely run out of materials, and weren’t even close to finishing the body.

Ventilation.

The Mi-To required more plates and electronics for the hull, but funding had already started to run dry. Problems with the cooling system and circuits delayed assembly by 9 months. During this time, the Division waited for more steel to arrive from Mitsubishi. The Sagami Arsenal had been contracted to supply the project with track and suspension assemblies. However, Sagami had not followed through with initial requests. The arsenal had also been asked to supply the 150mm steel plating used to construct the turrets for the vehicle, but the steel could not be allocated. Japan lacked steel for production, much less prototyping. This caused the turrets to be delayed for over a year. Within the first year of the projects start, the Technical Headquarters had considered to canceling the entire project multiple times. 

Mitsubishi managed to finish the Mi-To’s hull in February 1942, a month after construction restarted. The suspension and track links were built by this point and installed.  While the engine compartment had been worked on during late February, (1942), both side plates had distorted and collapsed due to the heavy rear armor and thin supporting side armor. This forced the engineers to place another 10mm steel wall in the engine compartment to support the weight of the tank. The 150mm plating for the front and rear of the Mi-To could not be built due to the lack of steel. The front and rear of the tank were to continue with single 75mm plates. An idea to produce an additional 75mm plate out of structural steel to make the prototype look as it would in production was proposed, but did not occur.

Prototype suspension.

On March 16th, a meeting was held by the Sagami Arsenal, Technical Headquarters, and engineers from Mitsubishi to discuss the design of the tank and the limited communication between all eleven members of the tank crew. Coordination in combat was a necessity. As such, a telecom system was to be installed in the prototype. Members of the cockpit, auxillary turrets, primary turret, and engine room could all phone-into each other to address orders and concerns that may arise during operation. The interior of the tank was to be. Crew could walk to and from each section of the vehicle by walking through two doors on the left and right sides of the tank. These sections were divided by two steel plates protecting the engine department, combat quarters, and drivers compartment.

On March 20th, the turrets began construction again but at a slow pace due to a lack of thick steel. In the meantime, a wooden mockup turret was built and was to be placed on the prototype with weights added to simulate the completed vehicle. The auxiliary turrets managed to finish in relative timing by June 4th.

Driver's periscope. 

The cockpit of the Mi-To was placed in the center of the chassis, sandwiched between the two 47mm auxiliary turrets. The driver’s vision was provided by a three sided periscope, each side measuring 178mm across. Operating the tank was difficult for the driver, as the clutch, brake, and transmission levers were far from their respective components at the rear of the tank. The Mi-To had a length of over ten meters, causing the tank to have low responsiveness to driver input. Therefore, a pressurized oil tube acting as a transmission assist had been installed in the center of the chassis, and the steering mechanism was given a hydraulic assist. The transmission, similar to that installed on the Type 97 chi-ha, had four forward gears and a single reverse gear. The transmission controls were placed in front of the driver and the steering handles at his sides. 

Driver's lever mapping.

Prototype engine RPM range.

The Mi-To was to be powered by two engines instead of one due to the excessive weight of the tank. Two Kawasaki V-12 800 horsepower engines were installed on the left and right sides of the engine room. The engine mounting layout of the vehicle resembled the design of the Type 89 I-Go. The driveshaft of the engines turn the cooling fan before going into the transmission. The transmission is connected to the multiple disc clutch between the two engines. The power is then transmitted to the final drive gear through the steering linkage extending to the left and right of the rearmost mechanism, and moves the drive wheel at the rear end of the tank. The two engines were originally supposed to be supercharged. However, the superchargers had too much parasitic power loss and produced excessive heat at high RPMs. The superchargers were removed, leaving the power output at 700HP per engine. This was further lowered as the cooling systems required 100HP out of each engine in order to keep the metal from melting. Final output at the wheels were 600HP per engine, 1200HP total.

The Mi-To could carry 400 liters of fuel. The fuel tanks were stationed at the top side of the hull at the rear. These were vulnerable to fire from the side of the tank, and during redesign the fuel tanks were to be relocated under the main turret and given a welded steel cover for protection. However this did not make it onto the prototype vehicle. 

Coolant line linkage to engine.

The construction of the suspension, hull and secondary turrets of the Mi-To finished in early 1943. The chassis weighed 47 tons empty without additional armour. The tank weighed 97 tons when configured for mobility and defensive tests, but this did not include the weight of the turrets. The Mi-To weighed 120 tonnes when the three auxiliary turrets and other pieces of armour were installed. The primary turret and additional 75mm plating would have given the tank a weight of 150 tonnes, just as designed. The vehicle had an overall height of 3.360 meters, width of 4.840 meters, and a length of 10.120 meters. Both tracks were 760mm wide. 

Tank Weight and additional armour plating weights ^

One project engineer the 47 ton chassis for mobility tests outside of the garage at the Mitsubishi facility. When driving with the first two gears, the Mi-To operated without any issues. Driving took place at the night, around tight street corners. This meant shifting into second gear was difficult, but there were no mechanical faults in the brief test. The speed topped out at 40kph on a straightaway. Major General Tomio Hara visited the site of the Mi-To’s construction before it was to begin official prototype trials and he had been pleased with the hydraulic suspension capability of the superheavy tank.

The Technical Headquarters decided disassemble the tank on May 26th to relocate it to the Sagami Arsenal, 51km from Tokyo. This process had all light plating removed from the tank alongside other equipment non-essential to the vehicle’s structure. These removable parts included;

Track Link

Upper deck plate, floor board plate, operating cabin desk, turret panels, turret room upper deck, tank hull rear upper plate, tank side plates, engine upper deck plate, engine room upper rear plate, upper track roller, lower roller wheel, suspension spring, spring covering, engine motors, engine starter, air filter, cooler, oil cooler, oil tank, fuel tanks, exhaust piping, muffler equipment, storage batteries, etc. 

Weights of varying parts on the tank ^

Removed parts were transported by truck to the Sagami Arsenal. All parts were covered by a wooden box at the rear of the trucks to hide components as to prevent citizens or potential spies from discovering the super heavy project. The main hull was placed on a 30 ton trailer, and towed by a 13 ton tracked towing tractor. The turrets were towed by Type 95 Ha-Go light tanks and a Type 97 Chi-Ha. Moving all the parts took 19 hours from Mitsubishi’s construction center to the arsenal. The tank did not get completely disassembled until June 9th. Once at Sagami, assembly began again on July 1st. Once completed on the 20th, the trials were scheduled to begin on the 1st of August.

Prototype chassis being towed.

On the day of the trial, high ranking officials in the Army were present to witness the first Japanese superheavy tank in motion. The Mi-To received a name change to O-I to follow Japanese naming convention (O translating to Heavy, I for First, making it "First Heavy") that was standard. Colonel Murata had been sent away for other purposes, and in his place was Lieutenant Colonel Nakano, Murata's assistant and colleague. Tomio Hara, head of the Sagamia Army Arsenal, was also present for the trials. 

Conditions for the trial were poor. Weather during the previous days caused the soil to soften, and terrain to become uneasy. The road at the proving grounds was narrow and when the O-I began moving down the road, the tank started to drift off the road and damaged the roadwork, resulting in the tank sliding off the road and into the soil. A change in the trial planning focused on the O-I traversing around in the dirt next to the facility it was kept in. Due to the surrounding buildings, its top speed could not be showcased, and traversing was difficult. The weight of the vehicle caused the land to start sinking in during pivots.  The tank managed to reach speeds of 18.7km/h at normal intervals. At full throttle, the tank could technically reach 29.4km/h.

driver's recollection of events.

The O-I reached the first three gears in its mobility test. At the end of the first day of the trials, the O-I had been put back into its garage and the engineers noticed that the seventh lower track roller’s bearing had been damaged, pinched between the drive sprocket and the outer plate rolling wheels come off on the right side of the tank were damaged, the roadwheels came loose, and it was caught between the drive sprocket and the outer plate skirt. The outer plate had broken through, and the gear of the drive sprocket broke apart into two pieces. 

Driver recollection of the chassis sinking into the mud.

The O-I had two bogies on each side, each bogie containing four wide roadwheels. The wheels were of cast construction and lacked rubber lining. During movement, the O-I chassis experienced severe rocking motions. Heavy duty shock absorber assemblies were placed inside the vehicle. The shock absorber was to connect the top and bottom of the vertical spring with two rods extending from the two front and rear swaying. One shock absorber supported the weight one bogie. The total ground pressure was 1.5 kg/cm^2 in soft terrain.

driver's recollection of events and damages sustained. 

American recalling of various Japanese
Prototype tanks.

From August 3 to August 8, the engineers began repairing the damage that had been done to the tank. 32 of the total 64 bearings were broken during the tests due to the bearings having insufficient strength. The excessive weight of the O-I caused the entire right side of the tank to collapse when at an extreme angle. Engineer Shigeo Otaka recalls the O-I undergoing immediate repairs to continue its trials. However, due to the steep cost of the repairs, they were put on hold for over a year, with the tank staying at Sagami Arsenal until March of 1945. The war had been nearing its end, and the usefulness of the O-I in Manchuria began to fade. The Soviet Union was no longer Japan’s primary enemy, and the development of more powerful anti-tank guns by the Soviet Union made the armour protection of the superheavy tank obsolete. The project to built a single prototype unit came at a steeper expense than originally thought possible, mass production was not realistically possible for Japan during the war. The project had thus been deemed a failure.

When the United States occupied Japan in late 1945 and began conducting Intelligence work at Japanese arsenals, an inquiry was made with the Sagami Arsenal specifically. The US force demanded details on the various prototype tanks located there to be sent with detailed specifications to the US forces in Japan and have the prototypes taken to the Aberdeen Proving Grounds in the US. Whether the Japanese scrapped the vehicle before the US arrived or if the United States managed to secure the hull and had it scrapped with other prototypes such as the Chi-Ri and Chi-To at Aberdeen remains unknown. All that is left is a single track link from the prototype. 

I have uploaded many of the original drawing schematics of the O-I to a single PDF file. As there were too many images to include in the article itself.