In a small shed at the top of a 100-foot-tall steel tower deep in the New Mexico desert, Donald Hornig sat next to the world’s first atomic bomb in the late evening of July 15, 1945, reading a book of humorous essays. A storm raged, and he shuddered at each lightning flash.

It was his second trip to the tower that day as part of the Manhattan Project, the secret American effort to build an atomic bomb. He had earlier armed the device, code-named Trinity, connecting switches he had designed to the detonators.

But J. Robert Oppenheimer, the scientific director of the project, had grown nervous about leaving the bomb alone. He told Dr. Hornig to return to the tower and baby-sit the bomb....