The security officer stopped the jeep at a nameless point in the Utah desert.

Maj. Charles Sweeney followed him out of the rig and watched as he reached down.

"He picked up a handful of sand and said we were working on a bomb that could turn a whole city into this," Sweeney recalled 50 years later.

Sweeney, a pilot, had arrived in Utah a few days earlier to start his new assignment as the training officer for a new top-secret squadron of B-29 bombers.

The isolated spot in the desert served as an open-air briefing room and Sweeney learned the nature of his mission.

The security man asked Sweeney if he had read an old Saturday Evening Post article about Albert Einstein and atomic energy. Sweeney remembered it.

The bomb used by the new 509th Composite Group would use atomic energy to blast a city to sand, the officer said.

Sweeney thought: "Oh boy, if we can drop all that with one airplane instead of a thousand, it's going to shorten the war."

Sweeney could talk about the atomic bomb only with the 509th's commander, Lt. Col. Paul Tibbets, in the presence of a security officer, and only in one secured room at the base in Wendover, Utah.

A test pilot who got hooked on flying during his first ride in a barnstorming biplane in his native Massachusetts, Sweeney had followed Tibbets to Utah. The two men first met in 1943 at an air base at Eglin, Fla.

Tibbets was a businesslike, no-frills veteran of the European theater, where he flew B-17s on bombing runs into Nazi territory.

Tibbets flew the world's second B-29 Superfortress to Eglin for tests. The first B-29 had crashed into a Seattle packing house, killing the crew and 19 workers on the ground.

The new B-29 "was the biggest thing we'd ever seen," Sweeney said.

It carried more bombs and flew farther, longer and higher than the B-17s and B-24s already in use. A B-17 carried 3,000 pounds of bombs; a B-29 carried 20,000 pounds. A B-17 struggled to 23,000 feet with a full load; a B-29 could easily reach 30,000 feet with a full load.

The B-29 was pressurized - like a modern airliner - so crew members did not have to use air masks at high altitudes.

As a test pilot, Sweeney probably had logged more flying hours in B-29s than anyone else.

At Wendover, the 509th crews knew they were training for a mysterious mission.

For months, they practiced dropping 10,000-pound, concrete-filled containers on targets from 30,000 feet.

New ballistic tables - mathematical charts plotting the course of dropping bombs - had to be calculated. These had to adjust to the mystery bomb's weird pumpkin shape and a bombing altitude higher than the normal 8,000 feet B-29s used on Japan.

Tibbets and Sweeney wanted more speed and height for the bombers. To do that, they had the B-29's machine guns stripped away, except for the tail guns.

It was a calculated risk. At 30,000 feet, they figured a Japanese fighter plane could realistically find and attack a B-29 only from the rear. At that altitude, though, the fighter would have only about one second to shoot.

In May 1945, the 509th moved to Tinian, a tiny island roughly 1,500 miles south of Tokyo. Conditions were primitive: C rations, pup tents and washing socks in a combat helmet.

There also were more practice missions. And the crews flew a few combat missions to bomb the cut-off Japanese garrison on nearby Truk Island or Japan itself to get a feel for flying combat in the Pacific.

Their payloads were big pumpkin-shaped bombs packing an explosive called torpex.

U.S. Army Air Corps Gen. Curtis LeMay decided the atomic bomb would be dropped from a single high-flying B-29 that would be pretending it was a weather plane.

That was later expanded to a flight of three. A second plane would carry scientific instruments, and a third would take photographs.

In late July, a few crews were shown a film and photos of the Trinity bomb blast in New Mexico and were told they would drop such a "gadget" on Japan.

"I thought, Jesus Christ, it was incredible, unbelievable," said Lt. Fred Olivi, the young copilot of a B-29 dubbed Bock's Car.