Francis Birch (1903-1992) was an American geophysicist.
In 1942, Birch worked at the MIT Radiation Lab, which was tasked with radar development. One of his primary projects was the proximity fuze, a device that used radar to determine the exact height to detonate a bomb.
Later, Birch was transferred to Los Alamos to join the Ordnance Division under Captain William Parsons. He first worked on the Thin Man plutonium prototype, but switched to work on the Little Boy uranium bomb after the Thin Man project was scrapped. Birch was put in charge of overseeing the manufacturing process of the uranium bomb in 1944.
Birch traveled with Little Boy to Tinian to supervise its assembly and the loading of the bomb onto the Enola Gay. He also created the 'double plug' system, which allowed Little Boy to be armed in the air, so that if the Enola Gay crashed, there would not be a nuclear explosion.