 Over the subsequent two decades McChord Field grew to roughly 3,000 acres (12 km²), encompassing the northern tip of the 70,000 acre (280 km²) Ft. Lewis.It became independent of Ft. Lewis in 1947 following the creation of the Air Force under provisions of the National Security Act of 1947 and was subsequently named McChord AFB.In 1940, McChord Field became the headquarters of the GHQ Air Force Northwest Air District, with a mission for the defense of the Pacific Northwest and Upper Great Plains regions of the United States.