The code names for the weapons - "Fat Man" and "Little Boy" - stem from characters in the written stories of writer Dashiell Hammett. Originally the names "Fat Man" and "Thin Man" were lifted directly from the stories, but the Thin Man weapon design (a Plutonium gun-type weapon) had to be abandoned. The relatively small Uranium gun-type weapon that followed was then named "Little Boy" as a contrast to "Fat Man".
Merriman's death was based on the deaths of two men: Harry Daghlian and Louis Slotin. On August 21, 1945 Harry Daghlian was fatally irradiated when conducting an experiment that involved stacking tungsten carbide bricks designed to reflect neutrons which could cause a subcritical mass to go critical. He lost his grip on a brick, which dropped in close proximity to the atomic pile (a stack of items that includes fissionable material; in this case a 3.5-inch-diameter sphere of plutonium). This resulted in an enormous burst of radiation. He tried to retrieve the brick but the exposure had already taken place. This initial dose might not have been fatal, but he was further exposed and suffered a severe burn to his hand while separating the pile, which was enough to prove fatal. He fell into a coma and died 25 days later at the age of 24. Precisely nine months later, on May 21, 1946, Louis Slotin was working with two half-spheres of beryllium suspended around a plutonium core (the same core that had killed Daghlian). He violated procedure by removing a pair of wooden shims separating the spheres, and using a screwdriver instead. The screwdriver slipped, causing contact between the half-spheres. There was a blue flash and a wave of heat. He jerked his hand upwards, causing the half-sphere to fall to the floor, ending the reaction. Neither Slotin nor the other scientists in the room were wearing dosimetry badges so no one is sure how much radiation was absorbed by each man. Four days later four of the eight men were discharged. One of the scientists sustained permanent neurological and vision problems. The other two recovered. Slotin died nine days after the accident, at the age of 35. Both men died from the same plutonium sphere. After their deaths it was given the name "demon core".
"Fat Man", according to the Atomic Heritage Foundation, "was the second plutonium, implosion-type bomb. The first was the 'Gadget' detonated at the Trinity site on July 16, 1945. In the implosion-type device, a core of sub-critical plutonium is surrounded by several thousand pounds of high-explosive designed in such a way that the explosive force of the HE is directed inwards thereby crushing the plutonium core into a super-critical state. Dropped on the Japanese city of Nagasaki on August 9, 1945, it was the second nuclear weapon used in a war".
"Little Boy", according to the Atomic Heritage Foundation, was a "...gun-type device, the critical mass is achieved when a uranium projectile which is sub-critical is fired through a gun barrel at a uranium target which is also sub-critical. The resulting uranium mass comprised of both projectile and target becomes critical and the chain reaction begins. Dropped on the Japanese city of Hiroshima on August 6, 1945, it was the first nuclear weapon used in a war".