 It was built by scientists and engineers at Los Alamos Laboratory using plutonium from the Hanford Site, and it was dropped from the Boeing B-29 Superfortress "Bockscar" piloted by Major Charles Sweeney.The name Fat Man refers to the early design of the bomb because it had a wide, round shape; it was also known as the Mark III.Fat Man was an implosion-type nuclear weapon with a solid plutonium core.