The bombs then called Blockbusters were the RAF's HC (High Capacity) bombs. These bombs had especially thin casings that allowed them to contain approximately three-quarters of their weight in explosive, with a 4,000-pound bomb containing over 3,000 pounds (1,400 kg) of Amatol. Most general-purpose bombs, termed Medium Capacity (MC) by the RAF, contained 50 percent explosive by weight, the rest being made up of the fragmentation casing. Larger Blockbusters were made later in the war, from the original 4,000 pounds (1,800 kg) version, up to 12,000 pounds (5,400 kg).
The Mark I 4000 lb bomb was a welded, cylindrical shell of 0.31-inch (7.9 mm) thick steel. The body of the bomb was 30 inches (76 cm) in diameter and 88 inches (2.24 m) long. The nose of the bomb was conical and a 27-inch (69 cm) long lightweight, empty cylindrical tail with a closed end was fitted, for a total overall length of 115 inches (2.92 m). A T-section steel beam was welded to the inner surface of the bomb to strengthen it. Subsequent Mark II and Mark III HC bombs differed in detail; the conical nose was replaced with a domed nose and the number of fuzes was increased from one to three to guarantee detonation. The Mark IV bomb did not have the T-section beam and the Mark V and Mark VI bombs were versions manufactured in the United States.
The larger 8,000 lb bomb was constructed from two 4,000 lb sections, of a larger 38 in (97 cm) diameter, that fitted together with bolts. A 12,000 lb version was created by adding a third 4,000 lb section.
The 4,000 lb high-capacity design was little more than a cylinder full of explosives: it was unaerodynamic and did not have fins. The same weight American 4,000 lb (1.81 tonne) AN-M56 general purpose bomb was aerodynamically designed as other US bombs were, with a sheet metal tailfin assembly and shaped nose and aft sections. When fitted with a nose spoiler and a drum tail the British "blockbuster" bomb fell straight down. These bombs were designed for their blast effect, to cause damage to buildings, specifically to blow roof tiles off, so that the small 4 lb (1.8 kg) incendiary bombs could reach the building interiors. The high capacity bombs were used only by the RAF, the only air force with bombers with bomb bays large enough to hold them.
In 1947 Alfred Cecil Brooks of Stourbridge was appointed a Member of the Order of the British Empire, for creating the Blockbuster, although his citation was worded "outstanding services to the King of a nature that cannot be revealed". The local newspaper referred to him as "Blockbuster Brooks".