The Tallboy or Bomb, Medium Capacity, 12,000 lb, was an earthquake bomb developed by the British aeronautical engineer Barnes Wallis

Barnes Wallis

Sir Barnes Neville Wallis, CBE FRS, RDI, FRAeS , was an English scientist, engineer and inventor. He is best known for inventing the bouncing bomb used by the RAF in Operation Chastise to attack the dams of the Ruhr Valley during World War II...

 and deployed by the RAF

Royal Air Force

The Royal Air Force is the aerial warfare service branch of the British Armed Forces. Formed on 1 April 1918, it is the oldest independent air force in the world...

 in 1944. It weighed five ton

Long ton

Long ton is the name for the unit called the "ton" in the avoirdupois or Imperial system of measurements, as used in the United Kingdom and several other Commonwealth countries. It has been mostly replaced by the tonne, and in the United States by the short ton...

s and, carried by the Avro Lancaster

Avro Lancaster

The Avro Lancaster is a British four-engined Second World War heavy bomber made initially by Avro for the Royal Air Force . It first saw active service in 1942, and together with the Handley Page Halifax it was one of the main heavy bombers of the RAF, the RCAF, and squadrons from other...

, was effective against hardened structures against which earlier, smaller bombs had proven ineffective.
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

Victory Bomber

The British "Victory Bomber" was a Second World War design by Barnes Wallis at Vickers-Armstrongs at Brooklands for a bomber to carry his projected "earthquake bomb" to strategic targets in Germany.-Background:...

" of 50 tons, which would fly at 320 mi/h at 45000 feet (13,716 m) to carry the heavy bomb over 4000 miles (6,437.4 km), but the Air Ministry were against a single-bomb aircraft and the idea was not pursued beyond 1942.

Following Wallis's 1942 paper Spherical Bomb — Surface Torpedo and the design of the "bouncing bomb

Bouncing bomb

A bouncing bomb is a bomb designed specifically to bounce to a target across water in a calculated manner, in order to avoid obstacles such as torpedo nets, and to allow both the bomb's speed on arrival at the target and the timing of its detonation to be pre-determined...

" for the Dam Busters

No. 617 Squadron RAF

No. 617 Squadron is a Royal Air Force aircraft squadron based at RAF Lossiemouth in Scotland. It currently operates the Tornado GR4 in the ground attack and reconnaissance role...

 of Operation Chastise

Operation Chastise

Operation Chastise was an attack on German dams carried out on 16–17 May 1943 by Royal Air Force No. 617 Squadron, subsequently known as the "Dambusters", using a specially developed "bouncing bomb" invented and developed by Barnes Wallis...

, the design and production of Tallboy was done without a contract on the initiative of the Ministry.