During 1938-39, the US military started making a new set of plans for possible war scenarios. These were the Rainbow series, as follows:
- Rainbow 1. To prevent violation of the Monroe Doctrine, and to protect the United States, its
possessions, and its sea trade.
- Rainbow 2. To carry out No. 1, and also to sustain the authority of democratic powers in the Pacific zones.
- Rainbow 3. To secure control of the western Pacific.
- Rainbow 4. To afford hemisphere defense, through sending U. S. task forces if needed to South America, and to the eastern Atlantic.
- Rainbow 5. To achieve the purposes of 1 and 4, also to provide ultimately for sending forces to Africa or Europe in order to effect the decisive defeat of Germany or Italy or both. This plan assumed U.S. co-operation with Great Britain and France
Those are from The Chief of Staff - Prewar Plans and Preparations volume of the US Army's official history of WWII, which can be freely downloaded here. It explains further:
The first four plans were eventually set aside. Numbers 2 and 3 (never fully
developed in detailed planning) were formally canceled by the Joint Board on 6
August 1941, by which time the recognition of Germany as the principal foe
made this cancellation obligatory. Although formal cancellation of Numbers
1 and 4 did not take place until 4 May 1942, much that they contemplated, such as
the taking over of British bases (by the old-destroyer transfer of 3 September
1940) and the progressive use of Atlantic sea patrols, was in effect long before
Pearl Harbor. Rainbow 1 and 4 were rendered obsolete by the fact that their major
premise was not fulfilled—that is, Britain's naval power was not neutralized, and
hence American's problem of hemisphere defense was not thus magnified. Contemporaneously with the American-British Staff Conversations (ABC) of early
1941, Rainbow 5 was expanded into War Department Operation Plan, Rainbow
5, and War Department Concentration Plan, Rainbow 5, (and corresponding
programs of naval responsibility). This grand composite was the basic plan in
readiness when war actually came in December 1941, the program having been
continuously restudied and amplified in the light of co-ordination with British
plans. By that time it specified the exact activities contemplated for protecting
coasts and bases and for offensive operations overseas, but it had been modified
little in fundamental concept since its drafting.
A later volume in the series, Global Logistics and Strategy, 1940-43, downloadable here, has an account of the ABC-1 talks between the British and American chiefs of staff in January 1941. There's a very revealing quote on p. 54:
As to how Germany
was finally to be defeated, the American
staff had as yet no definite ideas. Admiral
Stark's hints, the preceding November, of
massive land operations in Europe had
aroused little enthusiasm among the Army
planners, particularly his suggestion of repeating Wellington's exploits in Spain. A
WPD paper prepared late in January
reached the conclusion, among others
similarly pessimistic, that an invasion by
the historic route through the Low Countries would be dangerous folly. Army
thinking, in general, was oriented toward
the initial, not the later, stages of an
So it seems that in the event of a British defeat, the plan would have been Rainbow 1 and/or Rainbow 4, to make time for policy decisions.
The decision to order the B-36 would have had two linked causes. Firstly, developing a large bomber aircraft was obviously going to take a long time, and getting started as early as possible was necessary. Geography made the basic requirements obvious, so the concept of a very long-range bomber was unlikely to become obsolete.
Second, the US Army Air Corps wanted to demonstrate the power of strategic bombing. This concept had been developed between the wars, and provided a justification for an air force as an independent service, which the USAAC leadership very much wanted to achieve. The idea was that strategic bombing could win wars, by itself, quickly and decisively. This wasn't actually true with early WWII technology, although it became so at the end of the war with the advent of atomic weapons.
As it turned out, the B-36 wasn't a very good idea, because it tried to push older technology too far. Of course, it's hard to predict that before finding it out the hard way. Jet-powered bombers, such as the B-52, replaced it fairly rapidly, given the sunk costs of the B-36 fleet, and were far more practical.