Many of the B models were upgraded to D standards in 1941 at the San Antonio Air Depot. The RDF loop was replaced with the football. The radio room, waist and bottom gun positions were replaced with D positions. “Mary Ann” of the movie, “Air Force” was a converted B model. At least three were semi-converted with just the waist guns and RDF football done. These were 38-211, and the two Alaska Cold Weather Test Detachment aircraft, 38-215, and 38-216.
As an aside, the Academy boxart is bogus, as the Buenos Aires flight happened in 1938 whereas the first B flight happened in 1939. Wishful thinking.