 The United States Post Office had been running the air mail service since 1918 mainly using variants of the de Havilland DH.4 biplane.In 1925, it decided to modernize and placed an order with Douglas for a replacement aircraft based on the Douglas O-2 observation biplane.The company modified an O-2 by covering over the forward cockpit to make a mail compartment and moving the pilot into what had been the observer's cockpit.