Wallis presented his ideas for a 10-ton bomb in his 1941 paper "A Note on a Method of Attacking the Axis Powers", which showed that a very large bomb exploding deep underground next to a target would transmit the shock into the foundations of the target, particularly since shock waves are transmitted through the ground more strongly than through air.

Wallis designed the "Victory Bomber" of 50 long tons (51 t), which would fly at 320 mph (510 km/h) at 45,000 ft (14,000 m) to carry the heavy bomb over 4,000 mi (6,400 km), but the Air Ministry opposed a single-bomb aircraft, and the idea was not pursued after 1942.

Accomplishments of the Tallboy included the 24 June 1944 Operation Crossbow attack on La Coupole which undermined the foundations of the V-2 assembly bunker, and a Tallboy attack on the Saumur tunnel on 8–9 June 1944, when bombs passed straight through the hill and exploded inside the tunnel 60 ft (18 m) below the surface (stopping Panzer reinforcements reaching Normandy).

[1] The weight of the Tallboy (approximately 12,000 lb or 5,400 kg) and the high altitude required of the bombing aircraft meant that the Avro Lancasters used had to be specially adapted.

These triggered detonation after a pre-set delay, which gave the bomb sufficient time to penetrate the target before exploding.

[5] The bomb was aimed at the target during an operation and proved capable of penetrating deep into hardened reinforced concrete when it hit.

[citation needed] An alternative technique was to arrange detonation depth so that the crater broke the surface—useful for attacking railway marshalling yards and similar targets.

To increase penetrative power, a large, specially hardened, steel plug had to be precisely machined and mated to a recess in the nose of the bomb.

The final stage of explosive filling required that a one-inch layer of pure TNT be poured over the Torpex filling, followed by sealing the base with a 4 in (100 mm) layer of woodmeal-wax composite with three cylindrical recesses fitted with the explosive boosters and into which three chemical time-fuses were inserted when the bomb was armed.

[citation needed] Tallboys were not considered expendable, and if not used on a raid were to be brought back to base rather than safely jettisoned into the sea.

[38] In September 2019, a Tallboy bomb was found in the Piast Canal in northwest Poland near the town of Świnoujście and scheduled for defusing.

[39] In October 2020, the Tallboy detonated during a deflagration operation, but there were no reported injuries to divers nor any damage to the port infrastructure from the underwater explosion.

Development was started in late 1944 and plans were made to drop them on the island strongholds of the Pacific to aid in softening their defences before amphibious assaults.

During the Korean War a number of T-10s were converted to the radio-guided Tarzon bomb and were dropped by Boeing B-29 Superfortresses to destroy railroad bridges and reservoir dams.

[41] After the Korean War ended and the B-29 and B-36 bombers were retired, the United States Air Force no longer had an aircraft that could drop the M-121, and the bombs were put in storage.

One of the last of the World War II Tallboy designs was dropped during a Commando Vault mission to clear a landing zone for helicopters on a ridge during the 1969 Battle of Hamburger Hill in Vietnam.