boeingairtransport

This appears to be Boeing Air Transport's Boeing Model 80A, registered NC227M, c/n 1085. Construction Number "1089" belonged to NC231M, which this is not, as evidenced by the number on top of the starboard wing. The registration NC221M belonged to a Fairchild KR-34C.

Nice photo, but there are some problems with the Smithsonian's description and dating. There are no DH-4Bs in this photo; most of the aircraft shown are Douglas, not just the ones on the left. They all share the Douglas's distinctive vertical fin shape, quite different from the DH-4B's, and they are "single-bay" biplanes with one set of interwing struts per side; the DH-4B was a "twin-bay" biplane with two sets of interwing struts on each side. Also, the Douglas planes must be either M-3s or M-4s; the M-2 was only bought by Western Air Express, not by the Post Office. The Boeing 40 belonged to Boeing Air Transport, judging by the logo on the side. (Going by the tail number, it appears to be the one registered as NC285.) These details mean the photo must be a few years later than 1924, because at that time the Douglas and Boeing planes didn't exist. 1927 seems most likely: the Boeing 40 had entered service, but the U.S. Post Office hadn't yet stopped flying the mail itself.

Thank you, Colin! You are correct and we will redo the caption shortly.

Boeing 80