For the prototype, cost and weight favor the Hornet. Lots of parts in common with the Wasp, and cheaper to build; also approximately 200 pounds lighter. For the production B-17, the R-1820 is lighter and less complex than the R-1830 with similar horsepower.
Bob
Boeing may have started the design while still part of United Aircraft and Transportation Corp., which also included Pratt & Whitney. As this was a private venture, they probably decided to keep as much as possible "in house". For the production aircraft, the Army told the aircraft...