Toward the end of 1943, Manhattan Project scientists at a secret mountain laboratory complex at Los Alamos, New Mexico, began to see the final form of their new creations. The bombs that could reduce whole cities to radioactive rubble were so large and complex that they would have to be delivered by special bombers and specially trained crews. So secret was the training operation that the crewmen themselves weren’t told exactly what they were training to do. It was run out of a tiny border town in northwest Utah.
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Harvard physicist Norman F. Ramsey led the bomb delivery effort. (Ramsey would later share a Nobel Prize in physics for unrelated work.) "It was apparent," he wrote later in a paper about the project, "that the only United States aircraft in which such a bomb could be conveniently internally carried was the B-29…. Except for the British Lancaster, all other aircraft would require such a bomb to be carried externally unless the aircraft were very drastically rebuilt."
The Boeing B-29 Superfortress, which was first flown in September 1942, marked a quantum leap in bomber design. Because its fuselage was pressurized, crews could operate in shirt-sleeved comfort. A flight engineer tended the airplane’s systems, freeing the pilots to concentrate on flying. The big ships handled well, although the controls sometimes required a bit of muscle. But the B-29 was imperfect. In the labored climb to the stratosphere, the 2,200-horsepower Wright R-3350-13 Duplex Cyclone engines overheated dangerously. An engine fire, fed by the magnesium used to lighten the crankcase, could sever a wing.
Still undergoing tests at Eglin Field in Florida, the bomber would become operational in the summer of 1944.
In January of that year, on orders from General Henry H. "Hap" Arnold, commander of the Army Air Forces, a B-29 from Smoky Hill Army Air Field in Kansas arrived at what is now Wright-Patterson Air Force Base in Dayton, Ohio, to be secretly modified. Six thousand man-hours later, the airplane’s two 12-foot bomb bays had become a single bay 33 feet long, allowing the longer of the Los Alamos bomb designs to be tucked into the fuselage under the wing spar. The modified aircraft was to be the first of a very limited number of B-29s rigged for nuclear combat. The program was called Silver Plated, which was eventually shortened to Silverplate. There would be 65 Silverplate bombers in all.
In March 1944, the prototype Silverplate B-29 flew to California’s Muroc Army Air Field (now Edwards Air Force Base) to begin drop tests of full-scale dummies of the first two Los Alamos nuclear devices: the 17-foot-long Thin Man and the 10-foot-long, five-foot-diameter Fat Man. During these tests, a Thin Man replica escaped from its shackles before the bomb bay doors were opened, severely damaging the aircraft. "With this accident," Ramsey wrote, "the first Muroc tests were brought to an abrupt and spectacular end."
While the B-29 was repaired and the accident investigated, a team led by Navy Captain William S. "Deak" Parsons, inventor of the proximity fuse, worked on the detonation devices and bomb design. When the plutonium acquired for Thin Man was found to contain impurities that could trigger a premature detonation, the bomb was reconfigured to use uranium-235 instead. The new design, called Little Boy, was more compact—not quite 10 feet long and about two feet in diameter—and weighed about five tons. The Los Alamos scientists thought the bombs could be ready for combat by August 1945.
The prototype airplane, meanwhile, was restored to its two-bomb-bay configuration and re-fitted with the shackle and release mechanisms the British used to hang 12,000-pounders in the Lancaster. In September 1944, the prototype flew to the Glenn L. Martin modification center in Omaha, Nebraska, to serve as a template for bringing 24 B-29s up to the Silverplate configuration, which was still evolving.
At about the same time, Lieutenant Colonel Paul Tibbets Jr., 29, was chosen to command the tactical unit created to deliver the special bombs. A B-17 commander with combat experience over North Africa and Europe, Tibbets had spent a year as a B-29 test pilot and knew the big, temperamental bomber inside and out. He believed that, once over Japan, the airplane’s performance would be more important than its protection. So he stripped his bombers of their state-of-the-art fire-control system, and of all guns and armor except those in the tail. The change pared more than 7,000 pounds from the aircraft, adding several thousand feet of operational altitude and improving maneuverability. At 34,000 feet, these stripped-down B-29s could out-turn a P-47.
Even as the Silverplates gave up their guns, the first standard B-29s touched down on Saipan, an island in the Northern Marianas that U.S. forces had taken in June 1944. Seabees (the Navy’s Construction Battalions, or CBs) were soon turning Tinian, Saipan’s southern neighbor, into a vast complex of airfields. North Field, with four 8,000-foot runways, would eventually be the world’s biggest airport, with hundreds of B-29s leaving to strike Japan.