M110 (T14) "Grand Slam" 22,000-lb Bomb
M123 (T39) "Tallboy" 12,000-lb Bomb

During the Second World War, the British designer Barnes Wallis developed the largest conventional bombs used in combat. Wallis first designed the "Upkeep" bouncing bombs that were used during the Dambusters Raid on 16 May 1943. The rotating bouncing bomb exploded at the base of the retaining wall of the dam, producing heavy floods and damaging German production in the Ruhr.

Wallis next produced the 12,000-lb "Tallboy," also known as the "earthquake bomb." Tallboy was 21 feet long, with an overall diameter of 3 feet, 8 inches. The bomb body itself was 10 feet, 4 inches long and 3 feet, 2 inches in diameter. It weighed a total of 11,855 pounds, of which 5,200 pounds was Torpex D1 explosive. The weight of the case was thus a high proportion of the weight of the bomb. Dropped from 20,000 feet, a Tallboy made a 80 feet deep crater, 100 feet across. The bomb had a high terminal velocity, variously estimated at 3,600 and 3,700 feet a second (much faster than the speed of sound), and at these speeds it could go through 16 feet of concrete. It was used for attacks on tunnels, V-1 Flying Bomb launch sites, and other high-priority targets. Its most important use was in the sinking of the German battleship Tirpitz at anchor in Norway, on 12 November 1944. Over 700 Tallboys were dropped during the War.

In 1945, Barnes Wallis developed the 22,000 pound "Grand Slam," which remained the largest conventional bomb ever used in action. The 10 ton (22,000 pound) Grand Slam was 26-feet, 6-inches long. Its hardened casing was cast in a single piece in a sand mold, using a concrete core. The Grand Slam could reportedly penetrate though 20 or more feet of concrete. The first one was dropped on Germany on 14 March 1945. It hit the ground about 80 feet from the target, but it created a crater over 100 feet deep. This bomb was used with great effect against viaducts or railways leading to the Ruhr and also against several U-boat shelters. In one raid on 27 March 1945, against the U-Bootbunkerwerft "Valentin" submarine pens near Bremen, 2 Grand Slams penetrated 7 meters (23 feet) of reinforced concrete, bringing down the roof. In total, 41 Grand Slams were dropped during the war.

The US military expressed significant interest in these types of weapons, designating the Tallboy as the T39 and the Grand Slam as the T14. These were standardized as the M110 and M123 respectively. The US also developed an experimental weapon twice the size of Grand Slam, designated the T12, and a small 10,000 pound weapon, designated the T56 and standardized as the M121. These weapons provided conventional alternatives to nuclear ordnance in heavy bombers such as the B-36 in the 1950s.