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If we know for certain that Little Boy was actually considered for carriage by Lancasters, that will give us a bit better idea as to what might have been a legitimate target. Little Boy was bigger than Thin Man, which would have had an impact on the Lancaster's range etc; that we know from the figures provided here already, so what did those guys who considered the Lancaster have in mind for its use?
The Silverplate project was initiated in June 1943 when Dr. Norman F. Ramsey from the Los Alamos Laboratory's E-7 Group 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, or the oval shape of the Fat Man.[1]
Prior to the decision to use the B-29, serious consideration was given to using the British Avro Lancaster with its cavernous 33-foot (10 m) bomb bay to deliver the weapon. It would have required much less modification, but would have required additional crew training for the USAAF crews. 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.[2]
The first B-29 was delivered to the USAAF on 1 July 1943,[3] 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 out at the Naval Proving Ground in Dahlgren, Virginia, in August 1943. No aircraft was available that could carry the 17-foot (5.2 m) long Thin Man, so a 9-foot (2.7 m) 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.[3][4]
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.[5]
The 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.[1] 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 on 2 December.[8]
Modifications to the bomb bays of 42-6259 were extensive and time-consuming. Its four 12-foot (3.7 m) bomb bay doors and the fuselage section between the bays were removed and a single 33-foot (10 m) bomb bay configured. The length of the initial gun-type bomb shape was approximately 17 feet (5.2 m), necessitating that it be carried in the aft bomb bay, with some of its length protruding into the forward bay. The implosion-type bomb was mounted in the forward bay.[8] New bomb suspensions and bracing were attached for both shape types, and separate twin-release mechanisms were mounted in each bay, using modified glider tow-cable attach-and-release mechanisms.[9
So, to summarise, the Lancaster was initially considered to carry the A-bomb because of the size and shape of Thin Man.
There was a bomb bay access door at the Engineer's station, so it would have been possible.On the B-29 the bomb bay was accessible in flight. The Hiroshima Little Boy was in fact armed, that is the detonation charges installed, while in flight. Would this have been possible to do in a Lancaster?
I think that 'access' was just for visual checks. I think to actually fiddle with a bomb some further modification would be required (Grand Slam pictured).
View attachment 528584
It's possible that had the B-29 program suffered some calamity, the the US would have turned to it's slightly less advanced stablemate the B-32Dave, the Lanc doesn't have the range to fly from Tinian. Okinawa would be the only realistic option. RAF Tiger Force was looking at Okinawa as a base for ops with Lincolns and Lancs, but the A bombs were dropped before this plan got underway.
Tiger Force (air) - Wikipedia
The other problem with using the Lanc and indeed the Lincoln as nuclear bombers is the fact they could only carry the Little Boy. Fat Man was, well, too fat. This severely limits the flexibility of the mission. Fat Man ws ordered into production, Little Boy was initially unique, although after the war production on a limited scale was undertaken, but not to the same numbers as Fat Man. Also, timewise, the Lincoln entered service in August 1945 with 57 Sqn receiving 3 of them (!), so that rules it out of the equation if the original timeline of this is to be honoured.
Lets be frank here. To drop an atomic bomb on Tokyo, Japan, you only need a limited production run of a pressurised bomber that has to fly outward from Tinian 1500 miles and back to Iwo Jima another 750 miles to refuel, take off again and return to Tinian. That's 2250 miles on the first leg, 3000 overall but not the 3000 there and back that the Superforts did. The only other pressurised high altitude bomber is the Vickers Windsor which had a range of 2890 miles carrying 8000 lbs of bombs when powered by Merlin 65's. As I see it, the Windsor is the only viable alternative. You only need a limited production run, so no problem.It's possible that had the B-29 program suffered some calamity, the the US would have turned to it's slightly less advanced stablemate the B-32
The B-32 most likely would not have been structurally able to make the demanding turn and dive
The Douglas B-19A had the ability to lift a max. load over 37,000 pounds with a max. ceiling of 39,000 feet. It's top speed was 265mph and had a range of 4,200 miles (with a load of 18,000 pounds).
Not sure of the actual dimensions of the bomb bay, but the B-19 was huge.Gee, that's some performance. How big was the bomb bay?
Gee, that's some performance. How big was the bomb bay?