The M103 Heavy Tank was the result of a US policy immediately post-WW II to develop a heavy tank capable of dealing with the current and suspected heavy IS series of Russian tanks.
Much of its automotive base was derived from the M46/47/48 series, but with a 120mm gun to cope with anything else on the battlefield at much longer rages than the 90mm guns of the Patton series could reach. The gun was a development of a wartime anti-aircraft gun and fired large, fixed case shells. The sheer size and weight of these shells necessitated the inclusion of a second loader, bringing the crew up to 5.Also interested in this tank, the USMC were looking for a heavy support vehicle for its infantry.
Virtually ordered off the drawing board in 1949, the initial production batch of the new tank got off to a poor start. Designated T43E1, about 300 were produced and almost immediately put into storage pending the correction of a series of issues. The modified tanks were finally designated M103 and received into US Army and USMC service in 1957. Throughout its relatively short life, the USMC was really the major client for the M103, demanding and funding various modifications such as turret changes and a computerised fire control system. These latter changes became the M103A1, with some being handed over the Army to cover gaps in their ranks.
The Army finally retired the M103 in the early ‘60s whereas the USMC continued with it through until 1972. Its final version had the diesel engine used in later M48s and the M60, which greatly extended its range, although it still remained underpowered by conventional standards.
The M103 was the last of its line. Like the British Conqueror, the Heavy Tank concept was overtaken by the rise of the Main Battle Tank, with the necessary firepower and armour to undertake the tasks originally assigned to these heavyweights.