The T7 Light Tank/ M7 Medium Tank was originally designed to replace the M3 Light Tank and M5 Light Tank, but it outgrew its original purpose and was accepted for production as the M7 Medium Tank before being cancelled. By the autumn of 1940 the US Armored Force was beginning to look at replacing the existing M2A4 and M3 Light Tanks, both of which were armed with a 37mm gun, carried fairly thin armour and had little potential to be upgraded.
In January 1941 the Armored Force issued a more detailed specification for the new vehicle. It was to weight around 14 tons (short), have a low profile, be armed with a 37mm gun and carry up to 38mm of armour.
The Rock Island Arsenal produced two designs, the T7 and the T7E1. The basic layout was the same on both. The new tank had sloped frontal armour, curved sponsons over the tracks and a smoothly curved turret. They differed in details. The T7 was to have a welded hull, cast turret and vertical volute suspension (as used on the M3 and M5 Light Tanks). The T7E1 was to have horizontal volute suspension, a cast/ welded turret and a riveted hull and superstructure. The new tank was longer and wider than the M3 family, not having suffered from the same width restrictions as the earlier design, but it was slightly lower, giving it the required low profile.
Neither of these original prototypes were completed. Riveted construction was abandoned during 1941 and so the T7E1 was cancelled. Work on its chassis was already quite advanced, and it was later used for suspension and transmission tests. A wooden mock-up of the T7 was built, and in the autumn of 1941 the Ordnance Department decided to move on to three new prototypes.
The T7E2 was to have a cast hull and turret and Wright R-975 engine. The T7E3 had a welded hull and turret, twin Hercules diesel engines and automatic transmission. The T7E4 was to have a welded hull and turret and the same combination of twin Cadillac engines and Hydro-matic transmission, already under consideration for use in the M5 Light Tank. These three prototypes would have been around 16 tons in weight.
After some further development work the T7E2 was accepted as the most promising of the three designs. The design was officially approved in December 1941 and work began on a single prototype. While the tank was under production it was decided to arm it with the same 57mm gun then being used on the Canadian Ram tank. This was a version of the British 6pdr gun. The turret ring from a Ram was combined with a modified T7 turret and this version of the tank was completed in June 1942.
At this stage the T7E2 performed well, and would have been a useful replacement for the M3 and M5, but the Armored Force made further changes. As the T7E2 prototype was being completed they requested that it be up-armed to carry a 75mm gun, and asked for thicker armour. With the new turret the tank now weighted 25 (short) tons, making it a medium tank.
In August 1942 the T7 was standardized as the M7 Medium Tank. International Harvester Co was awarded a contract to produce 3,000 M7s, with the first to be delivered in December 1942.
The first production-standard M7 reached the Armored Force at Fort Knox in December 1942. It proved to be very disappointing. Fully loaded it weighted 29 (short) tons, but still had the same engine as when it was a light tank. It was thus badly underpowered giving it poor performance. In an attempt to fix this problem work began on the M7E1, which was to use a Ford V-8 engine, but the Ordnance Department began to worry about the duplication of effort involved in producing two medium tanks. The M4 Sherman was now in full production and didn’t have the same problems as the M7. In February 1943 production of the M7 was cancelled, after seven had been completed. Work on the M7E1 continued for a little longer, but was cancelled in July 1943.
The failure of the M7 meant that the US Army had to begin work on a new light tank. The eventual solution was the M24 Chafee, but this didn't enter service in large numbers until late in 1944, leaving the M5 Stuart to fight on in British and American service despite being badly under-gunned and under-armoured.
Stats (with 57mm gun)
Production: 7 plus prototype
Hull Length: 17ft 7in
Hull Width: 9ft 2in
Height: 7ft 4in
Engine: 400hp Wright R-975
Max Speed: 35mph road, 18mph cross country
Armament: 57mm gun and two machine guns
Armour: up to 63mm