Little Boy and Fat Man.
These weapons were, however, as different as their monikers imply. Little Boy was a uranium, gun-type weapon, whereas Fat Man was a plutonium, implosion-style weapon. Two types were needed because there was only enough uranium for one gun-type weapon, and the U.S. government knew it would need to make more than one weapon. The insurmountable challenges associated with a plutonium gun-type design prompted the shift to the implosion-style weapon.
Creating atomic bombs
Consider the setting of the Second World War. Amid the impending resolve to end the war and stop the daily death toll of thousands, the scientists, engineers, and military personnel at Los Alamos intensely collaborated over 27 months to build two types of bombs.
Wartime troubles were unavoidably linked to practical concerns, such as fitting bombs into the B-29 bomb bay to carry the weapons into combat and building components that could tolerate the harsh conditions of high-altitude falls. Developing bombs was not a routine engineering project, according to Critical Assembly: A Technical History of Los Alamos during the Oppenheimer Years, 1943–1945. Many gaps existed in the scientific information needed to create the atomic bombs. It would require a full understanding of nuclear physics, chemistry, explosives, and hydrodynamics.