In the spring of 1942 the Nazis were building reinforced concrete submarine bases that were bomb-proof against any existing weapons. At this time the largest penetrating bomb available to Bomber Command weighed a mere 1000 pounds.
As early as the fall of 1940, the brilliant Vickers Armstrong engineer and designer of the Wellington Bomber, Barnes Wallis, had demonstrated in his research how extra-large, penetrating bombs could create an earthquake like pressure wave that would destroy nearby structures by displacing their foundations. Wallis was envisioning a weapon weighing 20,000 pounds that would be dropped from an altitude of 40,000 feet, and reach the speed of sound. Known as the "Tallboy," there was no aircraft in the foreseeable future that could carry it so the design was put on hold.
Following the success of Wallis's "Bouncing Bomb" in the Dambusters Raid, his Tallboy design was reviewed. There was still no aircraft capable of carrying the original design to 40,000 feet but the Lancaster was now operational and had proven itself able to carry a 12,000 pound weapon. Wallis revised his design, including offsetting the tailfins by five degrees. This improved the weapons stability significantly. Released from the optimum height of 18,000 feet the bomb took 37 seconds to reach the ground, impacting with a speed of 750 miles per hour. During the night of June 8, 1944 the first Tallboy was dropped, causing extensive damage to the Saumur Railway Tunnel, preventing enemy reinforcements including tank units from reaching the beaches of Normandy.
An example of the Tallboy's effectiveness occurred when they were used against the underground V-1 assembly and launch facilities at Wizernes, France. One caused a landslide that completely blocked an entrance to the underground storage area and on a second raid, additional landslides were caused that completely blocked the remaining four entrances. The earthquake-effect of the weapon was demonstrated on numerous other occasions such as when a "near miss" of sixty feet was sufficient to destroy the railway bridge at Bad Oeynhausen.
By the end of the war, a total of 854 Tallboys had been dropped on heavily reinforced V-1 and V-2 assembly and launch sites, submarine pens, tunnels, oil refining and storage sites, viaducts, canals, and bridges. Its most spectacular success was with the sinking of the Battleship Tirpitz.
The Tallboy was 21 feet in length and 38 inches at its maximum diameter. Its hardened steel case had a thickness of more than 4 inches in the nose. The tail of the bomb was made of aluminum. The weapon was filled with 5200 pounds of Torpex explosive and the actual detonation could be delayed up to sixty minutes.
During March, 1945 a larger version of the weapon, the 22,000 pound "Grand Slam" became operational. With a length of 25 feet, 5 inches and a diameter of 46 inches, this bomb required the complete removal of the Lancaster's bomb bay doors. A total of 41 Grand Slams were dropped during the closing days of the war.