Silverplate Silverplate was the code reference for the United States Army Air Forces ' participation in the Manhattan Project during World War II . Originally the name for the aircraft modification project for the B - 29 Superfortress bomber to enable it to drop an atomic weapon , Silverplate eventually came to identify the training and operational aspects of the program as well . The original directive for the project had as its subject line " Silver Plated Project " but continued usage of the term shortened it to " Silverplate " . Testing began with scale models at the Naval Proving Ground in Dahlgren , Virginia in August 1943 . Modifications began on a prototype Silverplate B - 29 known as the Pullman in November 1943 , and it was used for bomb flight testing at Muroc Army Air Field in California commencing in March 1944 . The testing resulted in further modifications to both the bombs and the aircraft . Seventeen production Silverplate aircraft were ordered in August 1944 to allow the 509th Composite Group to train with the type of aircraft they would have to fly in combat , and for the 216th Army Air Force Base Unit to test bomb configurations . These were followed by 28 more aircraft that were ordered in February 1945 for operational use by the 509th Composite Group . This batch included the aircraft which carried out the atomic bombing of Hiroshima and Nagasaki in August 1945 . Including the Pullman B - 29 , a total of 46 Silverplate B - 29s were produced during and after World War II . An additional 19 Silverplate B - 29s were ordered in July 1945 , which were delivered between the end of the war and the end of 1947 . Thus , a total of 65 Silverplate B - 29s were made . The use of the Silverplate codename was discontinued after the war , but modifications continued under a new codename , Saddletree . Another 80 aircraft were modified under this program . A last group of B - 29s were modified in 1953 , but never saw further service . Origin . The Silverplate project was initiated in June 1943 when Dr . Norman F . Ramsey from the Los Alamos Laboratory ' s Group E - 7 , identified the Boeing B - 29 Superfortress as the only airplane in the United States inventory capable of carrying either type of the proposed weapons shapes : the tubular shape of the Thin Man gun - type fission weapon , and the oval shape of the Fat Man implosion - type weapon . Prior to the decision to use the B - 29 , serious consideration had been given to using the British Avro Lancaster with its cavernous bomb bay to deliver the weapon . This would have required much less modification , but Major General Leslie R . Groves , Jr . , the director of the Manhattan Project , and General Henry H . Arnold , the Chief of United States Army Air Forces ( USAAF ) , wished to use an American plane , if this was at all possible . The first B - 29 was delivered to the USAAF on 1 July 1943 , and Groves met with Arnold later that month . Groves briefed Arnold on the Manhattan Project , and asked for his help in testing the ballistics of the Project ' s proposed bomb shapes . Arnold and the head of the Ordnance Division at Los Alamos , Captain William S . Parsons , arranged for tests to be carried at the Naval Proving Ground in Dahlgren , Virginia in August 1943 . No aircraft was available that could carry the long Thin Man , so a scale model was used . The results were disappointing ; the bomb fell in a flat spin . But the need for a thorough test program was demonstrated . Groves met with Arnold again in September 1943 . He informed Arnold that there was now a second bomb shape under consideration , the Fat Man , and formally requested that further tests be carried out , that not more than three B - 29s be modified to carry the weapons , and that the USAAF form and train a special unit to deliver the bombs . Arnold delegated responsibility for this to Major General Oliver P . Echols . In turn , Echols designated Colonel Roscoe C . Wilson as the Project Officer . Codename . Originally the name for the aircraft modification project for the B - 29 to enable it to drop a nuclear weapon , Silverplate eventually came to identify the training and operational aspects of the program as well . The airplane modification project fell under the purview of the Manhattan Project ' s Project Alberta after March 1945 . The original codename for the project was " Silver Plated " but continued usage of the term shortened it to the one word " Silverplate " . For security reasons , the codename " Silverplate " was not officially registered . Confusion then resulted when the War Department allocated " Silverplate " to another project . Arnold ' s office had to order the other agency to discontinue its use of the codename . Los Alamos ' s Thin Man and Fat Man code names were adopted by the USAAF for the weapons . A cover story was devised that Silverplate was about modifying a Pullman car for use by President Franklin Roosevelt ( Thin Man ) and Prime Minister Winston Churchill ( Fat Man ) on a secret tour of the United States . Initial phase . USAAF sent instructions to its Army Air Forces Materiel Command at Wright Field , Ohio , on 30 November 1943 , for a highly - classified B - 29 modification project . The Manhattan Project would deliver full - sized mockups of the weapons shapes to Wright Field by mid - December , where Army Air Forces Materiel Command would modify an aircraft and deliver it for use in bomb flight testing at Muroc Army Air Field in California . B - 29 - 5 - BW 42 - 6259 ( referred to as the " Pullman airplane " from an internal code name assigned it by the Engineering Division of Army Air Forces Materiel Command ) was delivered to the 468th Bombardment Group at Smoky Hill AAB , Kansas , on 30 November 1943 , and flown to Wright Field , Ohio , on 2 December . Modifications to the bomb bays were extensive and time - consuming . Its four bomb bay doors and the fuselage section between the bays were removed and a single bomb bay configured . The length of the gun - type shape was approximately . New bomb suspensions and bracing were attached for both shape types , with the gun - type suspension anchored in the aft bomb bay ( although its length protruded into the forward bay ) and the implosion type mounted in the forward bay . Separate twin - release mechanisms were mounted in each bay , using modified glider tow - cable attach - and - release mechanisms . Testing of bomb shapes . The Pullman B - 29 flew in to Muroc on 20 February , and testing began on 28 February . Twenty four drops were carried out before tests were discontinued so that improvements could be made to Thin Man . The bombs failed to release immediately , frustrating calibration tests . In what turned out to be the last test flight of the series on 16 March , a Thin Man was prematurely released while the B - 29 was still " en route " to the test range and fell onto the bomb bay doors , severely damaging the aircraft . 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 used on the Lancaster to carry the Tallboy bomb . After repair of the Pullman B - 29 at Wright Field , testing resumed with three Thin Man and nine Fat Man shapes dropped in the last two weeks of June 1944 . High speed photographs revealed that the tail fins folded under the pressure , resulting in an erratic descent . Various combinations of stabilizer boxes and fins were tested on the Fat Man shape to eliminate its persistent wobble until an arrangement dubbed a " California Parachute " , a cubical tail box with fins angled at 45° to the line of fall , was approved . The Thin Man gun - type design was based on the fissibility of the very pure plutonium 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 plutonium 239 and plutonium 240 obtained was found to have a high rate of spontaneous fission . To avoid pre - detonation , the muzzle velocity would need to be greatly raised , making the bomb impractically long . Thin Man was therefore abandoned and the weapon was re - designed to use uranium 235 . The muzzle velocity required was much lower , reducing the barrel length of the resulting bomb , now code - named Little Boy , to less than . This allowed the device to fit into a standard B - 29 bomb bay , so the Pullman was modified to its original configuration with the rear bomb bay a standard B - 29 design . All subsequent Silverplates were also configured in this manner . The Pullman B - 29 was flown to Wendover and assigned to further drop testing in September 1944 with the 216th Army Air Force Base Unit until it was damaged in a landing accident in December . Wartime production versions . On 22 August 1944 , to meet the requirements of the USAAF group about to be formed to train in the atomic mission , a production phase of Silverplate B - 29s was ordered from the Glenn L . Martin Company ' s modification center at Omaha , Nebraska under the designation Project 98146 - S . The first three of these second increment Silverplate B - 29s were delivered to the USAAF in mid - October , and flown to Wendover Army Airfield in Utah . They were fitted with British single - point bomb releases mounted on a re - designed H - frame suspension rack fitted in the forward bomb bay , so that additional fuel tanks could be carried in the aft bay . A new crew position , called the " weaponeer station " , was created in the cockpit with a panel to monitor the release and detonation of the bomb during the actual combat drops . Fourteen production aircraft were assigned to the 393d Bombardment Squadron for training , and three to the 216th Army Air Force Base Unit for bomb drop testing . By February 1945 the seventeen aircraft of the second increment were themselves in need of upgrades , particularly those of the 216th Army Air Force Base Unit . Four of the planes assigned to the 393d Bombardment Squadron ( now part of the 509th Composite Group ) were transferred to the 216th 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 . The first five of this third increment , known as Project 98228 - S , also went to the test unit . The order totaled an additional 28 aircraft , with delivery of 15 designated combat models for the 393d beginning in April . The final eight were not delivered until after the two atomic bomb missions in August . Two were given to the 216th while the remaining six were assigned to the 509th at Wendover as replacements in case of loss of bombers operating from Tinian . Ironically , five of these six were lost in crashes over the next few years . The final wartime Silverplates incorporated all technical improvements to B - 29 aircraft , as well as the final series of Silverplate modifications that included Curtiss Electric reversible - pitch propellers , and pneumatic actuators for rapid opening and closing of bomb bay doors . The British F - type bomb release and G - type attachment were installed , along with dual electrical and mechanical bomb release mechanisms . Early model B - 29s were plagued by a host of engine problems , and the early Silverplate bombers were no exception . One was written off after being badly damaged as a result of an engine fire in February 1945 . The fuel - injected Wright R - 3350 - 41 engines in the later model bombers delivered in July and August 1945 were greatly improved and far more reliable . This was an important factor in the success of the 509th Composite Group ' s operations . The Fat Man and Pumpkin bomb s weighed over , so weight was a concern , even with the more powerful engines . Weight reduction was accomplished by removal of all gun turret s and armor plating . This work was done by the 509th Composite Group for the early Silverplate aircraft , but later models were delivered without them . These B - 29s represented a significant increase in performance over the standard variants . Flying at put them above the effective range of Japanese flak . Each Pumpkin bomb mission was conducted by a formation of three aircraft in the hope of convincing the Japanese military that small groups of B - 29s did not justify a strong response . This strategy proved successful , and Japanese fighters only occasionally attempted to intercept the 509th Composite Group ' s aircraft . One B - 29 incurred minor battle damage in operations . Silverplate operational units . Including the Pullman B - 29 , a total of 46 Silverplate B - 29s were produced during World War II . Twenty - nine of these were assigned to the 509th Composite Group during World War II , with 15 used to carry out the atomic bombings of Hiroshima and Nagasaki . An additional 19 Silverplate B - 29s were ordered in July 1945 , which were delivered between the end of the war , and the end of 1947 . By this time only 13 of the original 46 were still operational . Thus , a total of 65 Silverplate B - 29s were made , of which 32 were operational at the start of 1948 . Fifty - seven Silverplates were produced by Martin - Omaha and eight by Boeing - Wichita . Thirty - one were eventually converted to other configurations , 16 were placed in storage and later scrapped , and 12 were lost in accidents ( including four of the Tinian bombers ) . Two , " Enola Gay " and " Bockscar " , are today displayed in museums . The only other United States Air Force combat unit to use the Silverplate B - 29 was the 97th Bombardment Wing at Biggs Air Force Base in El Paso , Texas . In the summer of 1949 it received 27 of the aircraft from the 509th Bombardment Wing when the latter transitioned to B - 50D bombers . The re - equipment of the 97th Bombardment Wing was part of an expansion of the atomic strike force to ten wings during 1949 . Within a year all were converted to TB - 29 trainers . One other Silverplate B - 29 , on temporary assignment in the United Kingdom was converted into a weather reconnaissance aircraft ( WB - 29 ) and transferred to the 9th Bombardment Wing at Travis Air Force Base in California . The last Silverplate B - 29 in service as a nuclear weapons carrier was reassigned to another role in November 1951 , ending Silverplate after nearly eight years . Follow - up programs . Use of the codename Silverplate was discontinued in May 1947 . It was superseded by a new codename , " Saddletree " . This was applied to aircraft modifications only . Initially it was intended that Saddletree would refer only to modifications to the B - 29s , but it came to be used for modifications to B - 50 and B - 36 bombers as well . Saddletree modifications included a new bomb bay frame and hoist , and replacing the British FG bomb release with the newly - developed U - 1 pneumatic bomb release mechanism . Due to delays in the delivery of the B - 50s , Saddletree commenced in February 1948 . In all , some 6 , 000 hours of work was carried out on each of the 36 B - 50s by Sacramento Air Material Area . The Joint Chiefs of Staff issued a directive in January 1948 for the modification of 225 B - 29 , B - 50 , and B - 36 bombers to carry nuclear weapons , along with eight C - 97 aircraft to carry bomb assembly teams . There were already 32 Silverplate B - 29s in service with the Strategic Air Command , so the Air Material Command was directed to carry out Saddletree modifications on a further 80 B - 29s , 36 B - 50As , 23 B - 50Bs and 18 B - 36Bs . Unfortunately , the B - 36A could not carry nuclear weapons without major modifications . As the anticipated enemy was the Soviet Union , the aircraft also required " winterization " to allow them to operate from Arctic bases . The project included 36 B - 29s , and the modification of 36 others to have an air refueling capability . With the addition of the 80 Saddletree B - 29s , a total of 145 B - 29s were modified to carry nuclear weapons , and 117 of these were assigned to operational units . In October 1951 , in anticipation of a major build - up of the USAF to 95 groups , and of delays in the B - 47 program , the Air Material Command was ordered to modify a further 180 B - 29s then in storage to carry the Mark 4 , Mark 5 , Mark 6 , and Mark 8 nuclear bomb s . Modifications were carried out in Oklahoma City and Sacramento . By the time the aircraft were all delivered in September 1953 , the B - 29 was being phased out , and the modified aircraft went back into storage without seeing further service . Costs . In 1945 , a B - 29 bomber cost $ 782 , 000 . It cost $ 32 , 000 to upgrade an aircraft to Silverplate configuration , so the total cost of a Silverplate bomber was $ 814 , 000 , about $ in 2018 dollars . The total cost of the 65 wartime Silverplate project was therefore $ 53 million in 1945 dollars . Adding $ 7 million for logistics , this put the estimated cost of the Silverplate project at around $ 60 million 1945 dollars , about $ 2018 dollars .