In 1925, the US Post Office asked for a mail plane to replace the DH-4s that it was using then. The new plane would need to use the same engine as the DH-4. A lot of these could be bought. The Boeing Model 40 could carry up to of mail. The mail was stored in two places at the front of the plane. The wings were made of wood. The Model 40 had landing gear that did not move.