Jacob Beser was a lieutenant in the United States Army Air Forces who served during World War II. Beser was the radar specialist aboard the Enola Gay on August 6, 1945, when it dropped the Little Boy atomic bomb on Hiroshima. Three days later, Beser was a crewmember aboard Bockscar when the Fat Man bomb was dropped on Nagasaki. He was the only person to have served as a strike crew member of both of the 1945 atomic bomb missions.

"Fat Man" was the codename for the type of nuclear weapon the United States detonated over the Japanese city of Nagasaki on 9 August 1945. It was the second of the only two nuclear weapons ever used in warfare, the first being Little Boy, and its detonation marked the third nuclear explosion in history. It was built by scientists and engineers at Los Alamos Laboratory using plutonium from the Hanford Site, and one was dropped from the Boeing B-29 Superfortress Bockscar piloted by Major Charles Sweeney.