'Silverplate' was the US overall designation of the USAAF’s participation in the 'Manhattan' project, initially in regard to the modification of Boeing B-29 Superfortress bombers to carry atomic bombs and to train their crews, but finally including the raids to drop single atomic bombs on the Japanese cities of Hiroshima and Nagasaki (October 1943/August 1945).
The programme culminated in World War II with the 'Silverplate' bombing of Hiroshima on 6 August and the 'Centerboard' bombing of Nagasaki on 9 August 1945 using the only two atomic bombs then in existence, though the USA expected to have another such bomb ready for use in the third week of August, with three more in September and a further three in October.
The 'Silverplate' programme was launched during October 1943 when Dr Norman F. Ramsey, a member of the Los Alamos National Laboratory Group E-7, identified the B-29 as the only US warplane capable of carrying either of the two proposed weapons shapes: the tubular 'gun-type' fission weapon shape and the oval plutonium implosion weapon shape. Furthermore, because the attachment box for the B-29’s main wing spars was located between the weapons bays, the gun-type weapon could have a maximum diameter of only 2 ft 0 in (0.61 m). Before the decision to use the B-29, serious consideration had been given to using a British heavy bomber, the Avro Lancaster, which would have required considerably less modification, but the idea was vetoed by Major General Leslie T. Groves, head of the 'Manhattan' project, who thought it 'beyond comprehension to use a British plane to deliver an American A-bomb'.
On 30 November 1943, the USAAF issued the relevant order to the Army Air Forces Materiel Command at Wright Field, Ohio, for a high-priority but also very highly classified B-29 modification project. The 'Manhattan' project would deliver full-size mock-ups of the weapon shapes to Wright Field by mid-December, where the AAFMC would modify an aeroplane and deliver it for use in bomb flight testing at Muroc Army Air Field, California.
A single B-29-5 (often quoted as the 'Pullman airplane' from an internal codename assigned it by the Engineering Division of AAF Materiel Command) was delivered to the 468th Bomb Group at Smoky Hill AAB, Kansas on 30 November 1943 and flown to Wright Field, Ohio, on 2 December. The modification of the weapons bays were extensive and time-consuming. The four weapons bay doors, each 12 ft 0 in (3.66 m) long, and the fuselage section between the bays were removed to allow the creation of a single weapons bay 33 ft 0 in (10.06 m) long: the gun-type shape was some 17 ft 0 in (5.18 m) long. New bomb suspensions and bracing were attached for both shape types, with the 'Thin Man' gun-type suspension anchored in the rear weapons bay, though its length protruded into the forward bay, and the 'Fat Man' implosion type mounted in the forward bay. Separate twin-release mechanisms were mounted in each bay, using modified glider tow-cable attach/release mechanisms. To document the tests, motion picture camera mounts were installed in the rear bay. The modifications were made using the dummy bomb shapes as models, and the gun-type shape proved to be a very close fit. All modifications were made by hand and the process required more than 6,000 man hours of labour and was not completed until February 1944.
Engine problems systemic to the B-29 delayed delivery of the 'Pullman' B-29 for flight testing until 20 February 1944. The first test drop at Muroc on 6 March involved a 'Thin Man', followed on 14 March 14 by two drops of a 'Fat Man' fitted with a ring-tail stabiliser designed by engineers at the National Bureau of Standards. The 'Thin Man' shape performed without major problems, but the 'Fat Man' shapes revealed major instability, apparently as a result of poor workmanship and misalignment. All three bombs had also failed to drop immediately, frustrating calibration tests. A fourth test flight resulted in the premature release of a 'Thin Man' shape while the B-29 was still en route to the test range and severely damaged the aeroplane. The modified glider mechanisms had apparently caused all four malfunctions, because of the weight of the bombs, and were replaced with British Type G single-point attachments and Type F releases as used on the Lancaster to carry the 12,000-lb (5443-kg) 'Tallboy' bomb.
After repair of the B-29 at Wright Field, testing resumed and three 'Thin Man' and nine 'Fat Man' shapes were dropped in the last two weeks of June 1944. Various combinations of stabiliser boxes and fins were tested on the 'Fat Man' shape to eliminate its persistent wobble until the approval of an arrangement dubbed a 'California Parachute', a cubical tail box with fins angled at 45° to the line of fall.
The 'Thin Man' design was based on the fissibility of the very pure Pu-239 isotope produced in microgram quantities by the Berkeley cyclotron. When the Hanford production reactors came on-line in the spring of 1944, the mix of Pu-239 and Pu-240 obtained was found to be much more fissile. To avoid pre-detonation, the impact velocity of the two masses of fissile material would have had to be greatly increased, making the bomb impractically long. The weapon was therefore redesigned to use U-235. The impact velocity required for a U-235 fission reaction is much lower, reducing the barrel length of the resulting bomb, now codenamed 'Little Boy', to less than 10 ft 0 in (3.05 m). This allowed the device to be loaded into a standard B-29 weapons bay. The 'Pullman' aeroplane was modified back to its original configuration with the rear weapons bay of the standard B-29 type. All subsequent 'Silverplate' aircraft were also configured in this manner.
The 'Pullman' B-29 was flown to Wendover Army Airfield, Utah, and assigned to further drop testing in September 1944 with the 216th Base Unit until it was damaged in a landing accident in December. On 22 August 1944, to meet the requirements of the USAAF group about to be formed for the atomic mission, a production phase of 'Silverplate' B-29 bombers was ordered under the designation Project 98146-S from the Glenn L. Martin Company’s modification centre at Omaha, Nebraska. In mid-October the first three of these second-phase 'Silverplate' B-29 bombers were delivered to the USAAF and flown to Wendover. They were fitted with British single-point bomb releases mounted on a redesigned H-frame suspension rack fitted in the forward weapons bay so that additional fuel tanks could be carried in the aft bay. A new 'weaponeer station' was created in the cockpit with a panel to monitor the release and detonation of the bomb during the combat drops.
Fourteen production aircraft were assigned to the 393rd Bomb Squadron and three to the 216th Base Unit for bomb drop testing. By February 1945 the 17 second-phase aircraft, particularly those of the 216th Base Unit, were themselves in need of upgrade. Four of the bombers assigned to the 393rd Bomb Squadron (now a squadron of the 509th Composite Group) were immediately transferred to the 216th Base Unit to meet an increase in its testing tempo. Rather than attempt to modify the existing aircraft a few at a time, a decision was made to start a new production series, and the first five of this third phase, known as Project 98228-S, also went to the test unit. The order totalled an additional 28 aircraft, with delivery of 15 designated combat models for the 393rd Bomb Squadron beginning in April. The last eight of the aircraft were not delivered until after the atomic bomb missions in August.
The final wartime 'Silverplate' bombers incorporated all the current technical improvements to B-29 aircraft, as well as the final series of 'Silverplate' modifications that included Wright R-3350-41 fuel-injected engines, Curtiss Electric reversible-pitch propellers, and pneumatic actuators for rapid opening and closing of the weapons bay doors. Weight reduction was also accomplished by removal of all gun turrets and armour plating. These B-29 bombers represented a significant increase in performance over the standard variants.
The 'Silverplate' operational mission was the bombing of Hiroshima on 6 August 1945. Hiroshima was the primary target, with Kokura and Nagasaki as the alternatives. The Enola Gay of the 393rd Bombardment Squadron, piloted and commanded by Colonel Paul Tibbets, commander of the 509th Composite Group, took off from North Field Air Base on the island of Tinian in the Mariana islands group, about six hours of flying time from Japan. The Enola Gay was accompanied by two other B-29 bombers, namely the Great Artiste, commanded by Major Charles W. Sweeney, with instrumentation, and an aeroplane later named Necessary Evil, commanded by Captain George Marquardt, with cameras to record the attack. After departing Tinian, the aircraft made their way separately to the island of Iwo Jima, over which they made rendezvous at an altitude of 8,000 ft (2440 m) and then set course for Japan.
The aircraft arrived over the target in clear visibility at an altitude of 32,335 ft (9855 m). During the journey, Captain William Parsons of the US Navy had armed the bomb, which had been left unarmed to minimise risk during take-off. His assistant, 2nd Lt Morris Jeppson, removed the safety devices 30 minutes before the bomber reached the target area.
About one hour before the bombing, Japanese radar detected the approach of US aircraft making for the southern part of Japan. An alert was given and radio broadcasting stopped in many cities, among them Hiroshima. At nearly 08.00, the radar operator in Hiroshima determined that the number of approaching aircraft was very small (probably not more than three) and the air raid alert was lifted. To conserve fuel and aircraft, the Japanese had decided not to intercept small formations. The normal radio broadcast warning was given to the people that it might be advisable to go to air-raid shelters if B-29 bombers were actually sighted, but no raid was expected beyond some sort of reconnaissance.
The release of the bomb at 08.15 (local time) went as planned, and the 'Little Boy' gun-type fission weapon with 130 lb (59 kg) of U-235 took some 43 seconds to fall to the predetermined detonation height about 1,900 ft (580 m) above the city. By this time the Enola Gay had flown 11.5 miles (18.5 km) before it felt the shock waves from the blast. As a result of crosswinds, the bomb missed its aiming point, the Aioi Bridge, by almost 800 ft (245 m) and detonated directly over the Shima Surgical Clinic with a blast equivalent to about 13 kilotons of TNT (the U-235 weapon was considered very inefficient, and only 1.38% of its material fissioned). The radius of total destruction was about 1 mile (1.6 km), with resulting fires across 4.4 sq miles (11.33 km˛).
Japanese officials determined that 69% of Hiroshima’s buildings were destroyed and another 6% to 7% were damaged. Between 70,000 and 80,000 people, or some 30% of the population of Hiroshima, died instantly and another 70,000 were injured. More than 90% of the doctors and 93% of the nurses in Hiroshima were killed or injured as most of these medical personnel had been in the area which suffered the greatest damage.
Although US aircraft had previously dropped leaflets warning civilians of air raids on 35 Japanese cities, including Hiroshima and Nagasaki, the residents of Hiroshima were given no notice of the atomic bomb. The Tokyo control operator of the Japanese Broadcasting Corporation noted that the Hiroshima station had gone off the air, and attempted to re-establish his programme by using another telephone line, but this too had failed. About 20 minutes later the Tokyo railway telegraph centre realised that the main line telegraph had stopped working just to the north of Hiroshima. From some small railway stops within 10 miles (16 km) of the city came unofficial and confused reports of a terrible explosion in Hiroshima. All these reports were transmitted to the headquarters of the Japanese army general staff. Military bases repeatedly tried to call the army headquarters in Hiroshima, and a young officer was ordered to fly immediately to Hiroshima, land, survey the damage, and return to Tokyo with reliable information for the staff. It was generally felt at headquarters that nothing serious had taken place and that the explosion was just a rumour.
The staff officer went to the nearest airfield and took off for the south-west. After flying for about three hours and while still nearly 100 miles (160 km) from Hiroshima, the staff officer and his pilot saw a great cloud of smoke from the bomb. In the bright afternoon, the remains of Hiroshima were burning. The aeroplane soon reached the city, around which it circled as those on board stared down in disbelief. A great scar on the land, still burning and covered by a heavy cloud of smoke, was all that was left. The aeroplane landed to the south of the city, and the staff officer, after reporting to Tokyo, immediately began to organise relief measures.
According to US sources, the immediate effects of the blast killed approximately 70,000 people in Hiroshima and 40,000 in the 'Centerboard' attack on Nagasaki three days later. Estimates of total deaths by the end of 1945 from burns, radiation and related disease, the effects of which were aggravated by lack of medical resources, range from 90,000 to 166,000. Some have estimated that as many as 200,000 had died by 1950 as a result of cancer and other long-term effects. Another study states that from 1950 to 1990 about 9% of the cancer and leukaemia deaths among bomb survivors was the result of radiation from the bombs. It is known that at least 11 prisoners of war died in the bombing.