The first Tri-Motor for the US Army Air Corps, an early 4-AT-A. Ex NC3444, it arrived at Wright Field on 8 February 1928, then officially designated as the XC-3. Remained in service until July 1932. By 1931 it no longer wore code P-506. The Army ordered seven 4-AT-Es as C-3As, later modified to C-9 standard. The sole C-4 was a 5-AT-B model and was followed by four 5-AT-Ds as C-4As. There was also a bomber prototype called XB-906, civil reg NX9652. Photo from: US Air Force