Answer
In 1942, Edwin McMillan joined the Manhattan Project, which was the top-secret U.S. government project to develop an atomic bomb during World War II. McMillan's research focused on the isotope uranium-235, which is fissionable, meaning it can be split apart to release a large amount of energy. McMillan was able to isolate uranium-235 in 1940, and he later played a key role in the development of the atomic bomb.