The arriving crewmen were told nothing about their mission, according to "Ruin From the Air," a 1977 history of the project by Gordon Thomas and Max Morgan Witts.
"Don't ask what the job is," Tibbets told his men. "Stop being curious. . . . Never mention this base to anybody. That means your wives, girlfriends, sisters, family."
To everyone's surprise, Tibbets granted everyone Christmas leave in December 1944. What they didn't know was that it was a ploy to test security. As the men of the 509th headed home, they were met at the Salt Lake City railroad station by undercover operatives posing as solicitous civilians and friendly servicemen.
Two men from the 509th answered the detailed questions of a friendly "officer" who said he would soon be joining the unit. Within a week, both men had been banished to a remote island off the coast of Alaska.
Crews made hundreds of practice runs over the Mojave Desert and the Salton Sea. The test bombs were full-sized mock-ups of the real thing -- the long and slender uranium "Little Boy" that would fall on Hiroshima and the bulbous plutonium "Fat Man" that would hit Nagasaki.
Most of the mock-ups were filled with concrete, but some contained everything but the nuclear components, including large quantities of conventional explosives in the triggering mechanisms.
On one Salton Sea run, a consulting engineer accidentally dropped one of the explosive Fat Man mock-ups too soon. Narrowly missing the town of Calipatria, Calif., the bomb buried itself in a hole 10 feet deep, but somehow failed to explode. Bulldozers were rushed to the scene to erase evidence of the accident.
On June 18, 1945, Truman approved military plans for the invasion of Japan. The initial assault, by 815,000 troops, would begin on the island of Kyushu on Nov. 1, followed five months later by an attack by 1.2 million troops on the island of Honshu. Gen. Douglas MacArthur said it could take 10 years to wipe out the last pockets of resistance, with total American losses reaching 1 million men.
Less than a month after Truman approved the invasion plans, the first atomic bomb was tested successfully at Alamogordo, N.M. Truman, realizing that he had an alternative to the invasion, was pleased, as was British Prime Minister Winston Churchill.
"The atomic bomb is the Second Coming in wrath," Churchill said upon hearing the news.
Believing that the Japanese should have one last chance to avoid the awesome power of the bomb, Truman issued an ultimatum: Surrender unconditionally or face "prompt and utter destruction." The Japanese ignored the demand, which made no mention of nuclear weapons.
Outmaneuvering some top officers who sought to take over the bombing mission, Tibbets rallied support from Washington to retain his command of the 509th and announced that he would pilot the plane that dropped the first bomb.
Forcing an unhappy Capt. Robert A. Lewis to accept the secondary role of co-pilot in what had been Lewis' B-29, Tibbets ordered his mother's name, Enola Gay, painted on the side of the fuselage.
Several hours before dawn on Aug. 6, 1945, the Enola Gay, lumbering under the load of the 9,700-pound bomb, struggled up off a runway on the island of Tinian for the 1,700-mile flight north to Hiroshima. Two other B-29s accompanied the Enola Gay to monitor the event.
Seventeen seconds after 8:15 a.m., from an altitude of 26,000 feet, bombardier Maj. Thomas Ferebee released the bomb. Tibbets, who carried poison pills for the crew in case the B-29 went down, put the plane into a sharp, diving turn to speed away from the imminent explosion.
At 8:16 a.m., 1,890 feet above the center of Hiroshima, the bomb detonated with a core temperature estimated at 50 million degrees.
"My God, what have we done?" Lewis wrote in his logbook.
The shock waves severely shook the retreating plane, but did not damage it.
Staff Sgt. Robert Caron described the view from his seat in the tail gunner's turret as "a peep into hell."
Tibbets looked back to see an immense mushroom cloud.
"It had already risen to a height of 45,000 feet, and was still boiling upward like something terribly alive," he wrote in his book. "Even more fearsome was the sight on the ground below. Fires were springing up everywhere amid a turbulent mass of smoke that had the appearance of bubbling hot tar."
The flight back to Tinian was uneventful, and Tibbets alighted from the plane to be awarded the Distinguished Service Cross. The medal was added to a collection that included the Distinguished Flying Cross, the Legion of Merit, the Air Medal and a Purple Heart he received for wounds suffered when his bomber was struck by cannon fire over Europe.
Tibbets' military career would continue for 20 more years. Although most of his assignments involved relatively routine desk jobs, his past sometimes haunted him.