United Airlines plane being loaded with airmail from a Model T Ford mail truck. United reaches all the way back to April 6, 1926, when they were founded as Varney Air Lines in Boise, Idaho. In 1930 they were bought by United Air Transport and in 1934 they and three other airlines merged to form United Airlines.
This attractive trailer that appears to be professionally built is by an unknown maker and is towed by what looks to be a postwar Pontiac.