The T-12 was a further development of the concept initiated with the United Kingdom's Tallboy and Grand Slam weapons: a hardened, highly aerodynamic bomb of the greatest possible weight designed to be dropped from the highest possible altitude in order to destroy hardened targets. The T-12 weighed 43,600 lb (nearly 20 metric tons), which was twice the size of the US's previous largest bomb, the American-built version of the British Grand Slam, the Bomb, GP, 22,000-lb, M110 (T-14). Only one plane, the Convair B-36 Peacemaker, was designed to carry the T-12, although a B-29 Superfortress was converted for testing. The T-12 was not a simple scale up of the M110 but incorporated modifications based on testing and calculations.

It is also important to clarify a further nickname imparted to this weapon — the Grand Slam bomb, which more correctly refers to the T-12's 22,000 lb (10,000 kg) predecessor. Additionally, "Grand Slam" was the name of a project to modify B-36 bombers to carry nuclear bombs, creating further confusion.

Weapons of comparable size to the T-12, such as the BLU-82 and GBU-43 Massive Ordnance Air Blast bombs, remain in the US inventory of superbombs, but their utility is limited outside the realm of psychological weapons and demolition. They are not hardened and so lack the hard target destruction capability of the T-12 and its cousins. Precision-guided munitions (or "smart bombs") have mostly removed the need for gigantic charges in air-dropped bombs. The T-12 itself was superseded by earth-penetrating nuclear weapons, which weighed much less but could achieve the same penetration.

See also

• Nuclear bunker buster
• Grand Slam bomb
• MOAB
• BLU-82