"Fat Man and Little Boy (a.k.a. Shadow Makers in the UK) is a 1989 film that reenacts the Manhattan Project, the secret Allied endeavor to develop the first nuclear weapons during World War II. The film is named after the weapons "Little Boy" and "Fat Man" detonated over Hiroshima and Nagasaki, respectively. The code names, originally Fat Man and Thin Man, were drawn from characters in the works of Dashiell Hammett. However, there's a possible secondary allusion to stout project director Gen. Leslie Groves and the slim scientific director, Dr. J. Robert Oppenheimer. The film focuses much attention on the frequently strained relationship between the two men."
https://en.wikipedia.org/wiki/Fat_Man_and_Little_Boy