The United States Post Office had been running the air mail service since 1918 mainly using variants of the de Havilland DH.4 biplane.

Small modifications were made to the exhaust system to keep fumes away from the pilot and the design was considered a success, but was not ordered into production.

When the CAM-3 (Chicago-Dallas) route was passed to National Air Transport Inc (NAT) in October 1925, the airline originally used the Curtiss Carrier Pigeon.

When the Post Office retired its aircraft in July 1926 after all the routes had been privatized, NAT bought ten M-3s and eight M-4s at auction.

With the introduction of three-engined aircraft types beginning in 1928, the Douglas mailplanes were withdrawn from service; some were sold but most were scrapped.