In 1925, behind the efforts of Representative Clyde Kelly, Congress passed "An Act to encourage commercial aviation and to authorize the Postmaster General to contract for Air Mail Service."  The Air Mail Act of 1925, sometimes known as the "Kelly Act" would allow for private companies to bid for airmail routes. The first of these Contract Airmail Routes was run by the Ford Air Transport and was between Detroit and Cleveland, using Stout 2-AT airplanes. 

A Ford Air Transport Stour 2-AT, Maiden Dearborn

By 1930, 34 Contract Airmail Routes would be established by 1930. Soon all airmail was shipped via private carriers and not the Post Office. Private airmail transport is credited with helping advance aircraft technology, as planes that were faster and bigger were built to deliver that mail and increase profits. Companies that had Contract Airmail Routes included Western Air Express, National Air Transport, Boeing Air Transport, Pacific Air Transport and Northwest Airlines. 

Charles Lindbergh flew Contract Air Mail Route 2 (CAM #2) for Robertson Aircraft Corporation before his famous Trans-Alantic flight (top)

He flew a commemorative flight tracing the route in 1928, carrying this letter (bottom-thanks to Bill Allen)

A Boeing Model 40 of Pacific Air Transport, which operated CAM #8, which had several stops on the West Coast

A Western Air Express Douglas M-2, which flew CAM #4, Los Angeles to Salt Lake, with a stop in Las Vegas

A National Air Transport For Trimotor. NAT operated 2 routes, CAM #3 (Chicago-Dallas) and CAM #17 (Chicago-New York)