Capital Airlines was an airline serving the eastern, southeastern, and mid-west of the United States. Capital's headquarters, crew training and aircraft overhauls were done at Washington National Airport across the Potomac river from Washington, D.C. In the 1950s Capital was the fifth largest United States domestic carrier by passenger count (and sometimes by passenger-miles) after the Big Four (American, United, TWA, and Eastern). Capital merged with United Airlines on June 1, 1961.