The Boeing 40B-2 was an important early air mail plane first introduced in the late 1920s. This Boeing Model 40B-2 flew airmail between Chicago and San Francisco and is one of the only two remaining Boeing Model 40 aircrafts. Initially, the government ran the earliest form of airmail service.

In 1927, Boeing Air Transport (BAT) introduced the Boeing’s Model 40B-2 and became the first privately operated airplane to provide transcontinental airmail and passenger service. With a large cargo area for both mail and people, it made air mail delivery faster and more dependable across the U.S. Later, BAT merged with several smaller companies to form the company known today as United Airlines.