Source: adapted from C. Allaz (2005) History of Air Cargo and Airmail from the 18th Century, London: Christopher Foyle Publishing.
The first regularly scheduled airmail service in the world began in 1918 on the New York-Philadelphia-Washington route, for which the letter rate was $0.24 (about $3.50 in 2006 dollars) with a frequency of one round trip per day. Several airmail services were developed in conjunction with mail train services, particularly when this permitted the mail to arrive in the morning in a city and could thus be delivered the same day. By 1921, a route spanning the United States had been forged. Due to the short range of the planes at that time, postal routes were composed of several stages, at most 375 km apart. Crossing the continent from New York to San Francisco involved 16 stopovers and took about 4 days, with some segments flown during the night. This represented a network structure typical of the first stage in the development of air services (basic linear services).
It is worth noting that the air route followed the transcontinental rail line, which was the main corridor of circulation of passengers and freight. From this corridor, it branched into feeder lines to distribute mail in specific regions. The initial development of postal air routes was assumed by the US Postal Service, which acquired planes and took care of the setting of airfields and air operations. Once the service was established and demonstrated to be economically feasible, it was contracted to private operators. By 1927 the conveyance of airmail was entirely privatized. Some of these contractors would become the first commercial airlines.