 Complete the following phrase:  Began its association with testing when it received the first of the five Boeing YB-9, first all-metal monoplane bomber aircraft designed for the United States Army Air Corps, on 14 September 1932. The new bomber proved impossible to intercept during air exercises in May 1932, strengthening calls for improved air defense warning systems. Received the first of 12 Boeing B-17 Flying Fortresses delivered to the U.S. Army Air Corps. Participated in good-will flights to Argentina, 1527 February 1938, Colombia, 312 August 1938, Mexico, 915 June 1939, and Brazil, 1028 November 1939. The trip to Buenos Aires represented the longest distance performance of its kind on record and won the 2nd Bombardment Group the Mackay Trophy in 1938. Following 24 January 