[First flight air mail cover addressed to William E. Boeing, Seattle, Washington, May 19, 1938]

First flight air mail cover addressed to William E. Boeing, Seattle, Washington, May 19, 1938.

Stamped cachet: "Aviation History, 1911-1938 - Glenn L. Martin Flies First Plane into Clay Center, Nebraska, 1911. 1911 - Pusher type plane. 1938 - Glenn Martin today. Martin and his plane. First Air Mail Flight, May 19, 1938."

Stamped cachet on verso: "First Air Mail Flight - Clay Center, Nebraska."