The first atomic bomb to be mass produced by the United States was the Mk-6. Designed for carriage by bombers only, it was intended for use against area targets. With a 40 kiloton warhead, the Mk-6 was carried by the B-29/B-50 Superfortress, B-36 Peacemaker, B-47 Stratojet, B-52 Stratofortress, and the Navy's AJ-1 Savage. Deployed in 1951, it was the first new strategic weapon since the Fat Man that was dropped on Nagasaki during World War II.
The Mk-6 had its nuclear materials in a special capsule that was inserted into the bomb unit by the bombardier before it was dropped. Designed as a safety measure, having the nuclear material in a separate capsule undoubtedly prevented several accidental detonations in the early 1950s.
Being an early fission weapon, the Mk-6 required a lot of nuclear material to produce its desired yield. With the advent of thermonuclear weapons in the late 50s it then became possible to have the same explosive yield with less material and the Mk-6 bombs in the inventory became a valued source of nuclear material for the newer generation of smaller weapons, with the last Mk-6 being withdrawn from service in 1957.
Source: Nuclear Weapons of the United States; An Illustrated History by James N. Gibson. Schiffer Publishing, 1996, p89.