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Let’s assume Britain did collapse and become a German occupied country in WWII. (Had Hitler and Goering not been such idiots, it might have happened)

The b-29 began development as an idea in 1938 for just such a possibility. An order was placed on aug of 1940, quite a bit before Pearl Harbor. Securing Iceland as a forward base was one of the first things we did, but it did also relieve England of this job so they could focus elsewhere.

https://en.wikipedia.org/wiki/Boeing_B-29_Superfortress

Boeing began work on pressurized long-range bombers in 1938, in response to a United States Army Air Corps request. Boeing’s design study for the Model 334 was a pressurized derivative of the Boeing B-17 Flying Fortress with nosewheel undercarriage. Although the Air Corps did not have money to pursue the design, Boeing continued development with its own funds as a private venture.[4] The Air Corps issued a formal specification for a so-called “superbomber”, capable of delivering 20,000 lb (9,100 kg) of bombs to a target 2,667 mi (4,290 km) away and capable of flying at a speed of 400 mph (640 km/h) in December 1939. Boeing’s previous private venture studies formed the starting point for its response to this specification.[5]

Boeing submitted its Model 345 on 11 May 1940,[6] in competition with designs from Consolidated Aircraft (the Model 33, later to become the B-32),[7] Lockheed (the Lockheed XB-30),[8] and Douglas (the Douglas XB-31).[9] Douglas and Lockheed soon abandoned work on their projects, but Boeing received an order for two flying prototypes, given the designation XB-29, and an airframe for static testing on 24 August 1940,