How cool is this? This map shows the nascent air mail routes across the United States back in the 1920s.
This first one is from April of 1926. Look how many stops you had to take from Boston to San Francisco!

Record Group: RG 28, United States Airmail Routes.
Rediscovery Identifier:11010
Source: U.S. National Archives
And here is the mail route map from August of 1928.

Record Group: RG 28, United States Airmail Routes.
Rediscovery Identifier:11011
Source: U.S. National Archives