Beyond the distinctive physical layout, the date makes this a Y1B-17. By February 1938 the Boeing 299 was already crashed, and the only YB-17A wouldn't fly for another two months. I believe Sabrejet is right about the serial - of the 13 Y1B-17s built, the first went to Wright Field (note the spearhead on the aft fuselage) and the rest to GHQAF's 2nd BG.