Why not the B-30 instead of the B-32 as the B-29 back-up option? With the post war upgrades, Connie soon became "Super Connie" that nearly met the ambitious 1940 USAAC range requirements for a Very Long Range super bomber.
The engineering development of the B-29 had been underway since mid-1938 when, in June 1940, the United States Army Air Corps requested a similar design from the Consolidated Aircraft Company in case of development difficulties with the B-29.
Boeing submitted its Model 345 on 11 May 1940, in competition with designs from Consolidated Aircraft (the Model 33, which later became the B-32), Lockheed (the Lockheed XB-30), and Douglas (the Douglas XB-31). Douglas and Lockheed soon abandoned work on their projects, but Boeing received an order for two flying prototypes, which were given the designation XB-29.