John Coster-Mullen (21 December 1946 – 24 April 2021) was an American industrial photographer, truck driver and nuclear archaeologist who played an important role in creating a public record of the design of the first atomic bombs.

He is known for his critically-acclaimed self-published book Atom Bombs: The Top Secret, Inside Story of Little Boy and Fat Man.

[5] Over the next forty years, he worked at a variety of jobs: at a camera store in Milwaukee; on inventory control at Beloit Corporation, a manufacturer of paper products; and photographing industrial equipment for Trane, an air conditioning company, Mercury Marine, a manufacturer of boat motors, and Pohlman Studios in Milwaukee.

[10][11] In 2004, he built, with his son Jason, a replica of Little Boy for the Historic Wendover Airfield Museum, the former training site of the 509th Composite Group in Utah.

[12] His most sensational discovery was that the Little Boy bomb was actually a "girl":[3] while most historians had described the bomb as working by firing a small, solid "projectile" of enriched uranium into a larger, hollow "target," Coster-Mullen established conclusively that the projectile was a hollow set of rings which contained the majority of the uranium, and that it was fired onto a narrow target "spike".

[3] He was an advisor to the National Atomic Museum, the Children of the Manhattan Project Preservation Association and the British Broadcasting Corporation, which used his work for a documentary.