Air travel and shipping were still relatively new and fresh in the 1920s and 30s. For the most part, Americans could only dream of taking trips from one city to the next, let alone from country to country. Airplanes were mostly limited to hauling mail and cargo, guns and bombs. This wasn't necessarily because air flight was expensive (though it wasn't cheap); it's because it took some years to develop civilian-friendly aircraft, companies to cater to those civilians, and a reliable infrastructure of airports and communications to make air travel and expanded shipping possible.
Whether for flying produce or people, however, the Japanese attack on Pearl Harbor and Germany's declaration of war against the US put the hammer to all but war-related flight.
The job of advertising during the war years was to keep concept, company, and commercialism alive for when the war ended. The Airlines Of The United States was a series of ads meant to depict "the near future" when the democracy of air power would return, in peace, for the "every man."
Illustrator James Bingham used different themes, historic and then current, to depict the dream and keep it in the hearts and minds of the American consumer.