"Thin Man" was the code name for a proposed plutonium-fueled gun-type nuclear bomb that the United States was developing during the Manhattan Project.

In 1942, prior to the United States Army taking over control of wartime atomic research in what became known as the Manhattan Project, Robert Oppenheimer held conferences in Chicago in June and Berkeley, California in July, at which physicists discussed nuclear bomb design issues.

[2] Oppenheimer reviewed his options in early 1943, and he gave priority to the gun-type weapon,[2] but as a hedge against the threat of predetonation, he created the E-5 Group at the Los Alamos Laboratory under Seth Neddermeyer to investigate implosion.

Oppenheimer led the design effort himself until June 1943, when United States Navy Captain William Sterling Parsons arrived and took over the Ordnance and Engineering Division and direct management of the Thin Man project.

As the head of the E-6 Projectile, Target, and Source Group, Critchfield calculated critical masses, and instituted a system of live testing with scale models using 20 mm cannon and 3-inch guns.

[9] The E-8 group estimated the muzzle velocity of the gun at around 3,000 feet per second (910 m/s), close to the maximum achievable in 1944,[11] and calculated that the pressure in the barrel would be up to 75,000 pounds per square inch (520,000 kPa).

A polonium-210-beryllium initiator was chosen because polonium 210 has a 140-day half life, which allowed it to be stockpiled, and it could be obtained from naturally occurring ores from Port Hope, Ontario.

To avoid predetonation or "fizzle", the plutonium "bullet" would need to be accelerated to a speed of at least 3,000 feet per second (910 m/s)—or else the fission reaction would begin before the assembly was complete, blowing the device apart prematurely.

[14] However, the American Boeing B-29 Superfortress could be modified to carry it by removing part of the bulkhead under the main wing spar and some oxygen tanks located between its two bomb bays.

Subscale models of the bomb were dropped from a Grumman TBF Avenger at the US Navy test range at Dahlgren, Virginia starting in August 1943.

"[22] Conant consulted Ernest Lawrence and Arthur Compton, who acknowledged that their scientists at Berkeley and Chicago respectively knew about the problem, but could offer no ready solution.

Conant informed the director of the Manhattan Project, Brigadier General Leslie R. Groves Jr., who in turn assembled a special committee consisting of Lawrence, Compton, Oppenheimer, and McMillan to examine the issue.

[24] The distance required to accelerate the plutonium to speeds where predetonation would be less likely would mandate a gun barrel too long for any existing or planned bomber.

All gun-type work in the Manhattan Project was directed at the Little Boy enriched uranium gun design, and almost all of the research at the Los Alamos Laboratory was re-oriented around the problems of implosion for the Fat Man bomb.