Big Bird

The B-36 Peacemaker, the largest bomber aircraft ever uilized by the United States military, was originally conceived early in  the Second World War, but did not become airborne until 1946.  It was the first long-range bomber put to use by the Strategic Air Command, a branch of the newly-formed U.S. Air Force charged with delivering a nuclear payload to the Soviet Union.

Fortunately, the B-36 never had to make a real delivery.