The US air Mail stamps appeared as early as 1918 with the inauguration of the Washington – Philadelphia – New York air link. A number of stamps were issued between 1918 and 1975 when the domestic air mail finally came to an end. Though, the international air mail continued till 1995.
The earliest stamps that I have of the US Air Mail are at top left corner above, which were issued in August 1947. I also have a set of four stamps in the fifth row that were issued in 1960.
I have more airmail stamps, which were issued subsequently, and will appear as I unfold my stamp album.
Related Link: List of United States airmail stamps (Wikipedia)