Because of limited budgets for tank development in the interwar years, at the outbreak of World War II the US Army possessed few tanks, though it had been keeping track of armor use in Europe and Asia. Successful employment of armored units in 1939 - 1940, mostly by the Germans, gave momentum to a number of US tank programs, including a heavy tank program. The United States possessed a massive industrial infrastructure and large numbers of engineers that would allow for mass production of tanks.
| M6 heavy tank | |
|---|---|
| Type | heavy tank |
| Place of origin |  United States |
| Service history | |
| In service | trials only |
| Used by | United States Army |
| Specifications | |
| Weight | 126,500 pounds (57.4 tonnes) combat loaded |
| Length | 27 ft 8 in (8.43 m) gun forward 24 ft 9 in (7.54 m) gun aft |
| Width | 10 ft 3 in (3.12 m) over track armor |
| Height | 7 ft 10 in (2.38 m) to turret roof 10 ft 7 in (3.23 m) to top of .30 machinegun mount |
| Crew | 6 (commander, gunner, driver, assistant driver, loader, assistant loader) |
| | |
| Armor | Front 3.25 in (83 mm) at 30° Upper sides 1.75 in (44 mm) at 20° Lower sides 2.75 in (70 mm) at 0° Turret front 3.25 in (83 mm) at 7° Turret sides and rear 3.25 in (83 mm) at 0° |
| Main armament | 1 × 3in (76mm) gun M7 (75 rounds) 1 × 37mm (1.46 in) gun M6 (202 rounds) |
| Secondary armament | 2 × Caliber .50 (12.7mm) Browning M2HB machineguns, hull (6,900 rounds) 2 × .30 Browning M1919A4 machine guns, one fixed (bow), one flexible AA (5,500 rounds) |
| Engine | 1,823 ci (29.88 liter) Wright G-200 9-cylinder gasoline 825 hp at 2300 rpm |
| Power/weight | 15.7 hp/tonne |
| Transmission | Timken mechanical model 16001, three speeds (two forward, one reverse); rear drive sprocket |
| Suspension | Horizontal volute spring |
| Ground clearance | 20.5 in (52 cm) |
| Fuel capacity | 477 US gallons (1,810 liters), 80 octane |
| Operational range | 100 mi (160 km) |
| Speed | 22 mph (35 km/h) |
Following the Chief of Infantry recommendation from 20 May 1940, the US Army Ordnance Corps started to work on a 50-ton heavy tank. Initially a multi-turreted design was proposed, with two main turrets armed with low-velocity T6 75mm (2.95") guns, one secondary turret with a 37 mm gun, and a coaxial .30 caliber (7.62mm) machinegun, and another secondary turret with a 20 mm gun and a coaxial .30 caliber machine gun. Four .30 caliber machineguns were to be installed in ball mounts, two in the glacis plate and two in the rear corners of the hull. The project was approved on 11 June 1940 and the vehicle received the designation Heavy Tank T1. The design was somewhat similar in concept to multi-turreted breakthrough tanks developed in Europe in the 1920s and throughout the 1930s, such as the British Vickers A1E1 Independent or the Soviet T-35. Disadvantages of these "land dreadnoughts", namely their excessive size, difficulty in coordinating actions of the crew, and high production costs, led to abandonment of the concept in Europe.
By October, the US developers reached the same conclusion as their European counterparts. The armament was changed to a single vertically-stabilized 3 inch (76.2 mm) gun and a coaxial 37 mm gun in a single three-man turret with both manual and electric traverse. The turret had a commander's cupola identical to that of the M3 Medium Tank. Additional armament consisted of two .50 caliber machine guns in a bow mount (operated by the assistant driver), two .30 caliber MGs in the front plate (fired electrically by the driver), one .30 caliber in the commander's cupola and one .50 caliber in a rotor mount in the right rear of the turret roof (operated by the loader). The crew consisted of commander (turret left), gunner (turret right), loader (turret), driver (hull left front), assistant driver (hull right front) and ammunition passer (hull).
One of the main challenges was developing a powerpack for such a heavy vehicle. The Wright G-200 air-cooled radial gasoline engine was selected by a committee formed by the Society of Automotive Engineers, but no suitable transmission was available. The committee recommended developing a hydramatic transmission, while a possibility of using a torque converter or an electric transmission was also to be checked.
In 1941 - 1942 three prototypes were built by the Baldwin Locomotive Works, one with electric transmission and two with torque converter transmission. Variants with hydramatic transmission were never completed. The prototypes also differed in hull assembly method - one had welded hull and two cast hull. On 26 May 1942 two variants with torque converter transmission were standardized as M6 and M6A1. Standardization of the electric transmission equipped T1E1 as M6A2 was never approved, but manufacturing of the vehicle was nevertheless recommended. It was proposed by the Ordnance Corps that 115 T1E1s would be built for the US Army and 115 M6s and M6A1s for US allies. The production started in December 1942. Some minor changes were introduced in the production vehicles: the cupola was replaced by a double-door hatch with a ring mount, the machine gun in a rotor mount and the left front machine gun were removed.
However by the time the M6 was ready for production, the Armored Corps had lost interest in the project. The advantages the M6 offered over medium tanks - its much thicker armor and slightly more powerful gun - were offset partly by the shortcomings of the design - such as very high silhouette, awkward internal layout and reliability problems - and partly by logistical concerns. By the end of 1942, the Armored Corps were sure that the new M4 Sherman gave adequate solution for the present and the near future, while being reliable, cheap and much easier to transport.
Work on M6 didn't stop at once. The T1E1 prototype was tested with a T7 90 mm gun and was found to be a satisfactory gun platform, although poor turret layout was noted again. In August 1944 the Ordnance Corps recommended using the T1E1s produced to build 15 77-ton vehicles designated M6A2E1, with thicker (up to 7.5-inch (190 mm) vertical protection) glacis armor and a turret developed for the T29 Heavy Tank, armed with a T5E1 105 mm gun. The proposal was rejected by General Eisenhower. However, by late 1942 main development effort shifted to other projects, one of which eventually resulted in the M26 Pershing.
On 14 December 1944 the M6 was declared obsolete. Only forty units were produced and they never left US soil. Several toured the United States for propaganda purposes, where they gave performance displays (such as car crushing) at War Bond drives and the like. All were eventually scrapped except for a single T1E1 which is on display at the United States Army Ordnance Museum, Aberdeen, Maryland.
Some suspension parts were used in the heavy Excelsior A33 Tank prototypes.
