Britain and Germany, amongst others, were also developing atomic bombs. You don't allow your enemies in a war know what you are doing!
ACDS
The Manhattan Project was the secret project to design and build the world's first atomic bomb.
The nuclear was a secret before the bombs hit the targets.
The development of the atomic bomb was not only the most ambitious scientific enterprie in history, it was also the best-kept secret of the war.
America's Secret Establishment was created in 1986.
America's Secret War was created in 2004.
President Franklin D. Roosevelt authorized the development of the atomic bomb, but Harry Truman was President at the time of the bomb's first successful test, and he is the one who authorized its use in populated areas. The Manhattan Project, the development of the atomic bomb, was such a secret that even Vice President Truman didn't know about it until after he became President. (I don't know whether or not Vice President Wallace knew about the project.)
its a secret
Actually there is no secret to the atomic bomb. However Klaus Fuchs & Ted Hall provided the plans of Fat Man. This saved them considerable engineering effort.
The atom bomb research and development were conducted under the Manhattan project.
It's a secret!
The atomic bomb dropped on Hiroshima during World War II was created by a team of scientists led by J. Robert Oppenheimer as part of the Manhattan Project. Oppenheimer is often considered the "father of the atomic bomb" due to his important role in its development.