It was the Me-264. But it was not to carry a dirty bomb. It was just to bomb New York. However I believe that if Hitler had succeded in getting the Atom Bomb like he wanted, the Me-264 would have carried it.
Type: Ultra Long Range Bomber
Origin: Messerschmitt AG
Engines:
Four 1,700 hp BMW 801D or G 18-cylinder radials
Dimensions:
Span: 43m
Length: 20.90m Height: 4.30m
Weights:
Empty 46,627lb.
Max. Loaded 123,460lb.
Performance:
Max. Speed: 565km/h (351mph)
Max range at 217mph 9,321 miles
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Comments
The Me 264 was designed from the begining as an 'Amerika' bomber. It's goal was to be able to carry a small load to the United States and return or a heavier load and ditch at sea and the crew picked up by U-boat.
The first prototype flew in December 1942, but soon allied presure forced Messerschmitt to slow development and the prototypes were eventaullay destroyed by Allied bombing.