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English: MK-4 Atomic Bomb was the first mass-produced nuclear weapon in the U.S. arsenal. Over 550 were built. This air-dropped weapon could only be detonated in an air burst, well above ground level, for maximum destruction of a city. The first box entered the arsenal on 19 March 1949, and the last was retired in May 1953.

It was the first nuclear weapon to be deployed overseas since WWII, when nine were sent to Guam during the Korean War in August 1950. The nuclear cores were later sent in April 1951, and plans were made to drop the bombs on seven Chinese and two Soviet cities in the far east by the 43rd Bomb Wing.

The MK-4 had a removable core which could be put in the bomb during flight. A well-trained weaponeer could insert the core and ready the bomb for combat use in less than 30 minutes. The uranium or plutonium core was inserted by removing the flat nose plate of the bomb, disconnecting the detonator cables, removing two detonators, removing a polar cap covering the forward hemisphere of high explosives, removing the outer and then inner tapered high explosive lenses, and pulling out a cylindrical plug made up of the aluminium pusher and the natural uranium tamper. This revealed the space for the core, an 11 cm diameter sphere of radioactive materials coated in nickel. The four high explosive lenses weighed about 156 kg; the pusher-tamper 4 kg; and the core nearly 6 kg.

The nuclear bomb components were designed by the Los Alamos Laboratory, and the exterior bomb shape was built by American Car & Foundry Company.