The M103 (formally designated as "Tank, Combat, Full Tracked, 120mm, M103") was a heavy tank design of American origination developed to counter the threat posed by the growing number of Soviet systems in use during the early stages of the Cold War. Development began during World War 2 though the project would not come to fruition until the end of the Korean War. The M103 managed a relatively short service life and saw service primarily with elements of the United States Marine Corps though the US Army proved the other notable operator in lesser numbers. No more than 300 units were ever completed and the rise of the Main Battle Tank ended the reign of the dedicated Heavy Tank for the foreseeable future. Designs such as the M103 undoubtedly paved the way for the MBT and proved the necessary stepping stone in the genesis of the modern fighting tank we know today. Many examples exist today as showpieces throughout the United States. A foreign example survived at the famous Bovington Tank Museum in the United Kingdom.
The massive tank clashes that permeated the World War 2 battlescape ushered in all-new developments during the conflict. For the Americans, its star performer became the ubiquitous M4 Sherman with its 75mm main gun, an armament not quite on par with contemporary offerings in Britain, Germany and the Soviet Union but nonetheless adequate. While Shermans were eventually handed the improved 76mm main gun, the M36 Tank Destroyer fielded the evermore powerful 90mm main gun in its open-topped turret and was designed exclusively as a tank destroyer. This proved timely with the arrival of considerably heavier, stouter and more powerful German tanks in the Panzer V "Panther", Tiger I and Tiger II series - the latter two classified as heavy tanks. The Americans were therefore forced to develop heavy-class tanks in turn to help counter the improved armor of the enemy. Additionally, advancing into German-held territories would also call into play fortification-defeating gunnery platforms which was also under consideration by the Allies on a variety of levels.