Capital Airlines was an airline serving the eastern, southern, southeastern, and midwestern United States. Capital's headquarters were located at Washington National Airport (now Reagan Airport) across the Potomac river from Washington, D.C. where crew training and aircraft overhauls were also accomplished. In the 1950s Capital was the fifth largest United States domestic carrier by passenger count (and sometimes by passenger-miles) after the Big Four air carriers (American, United, TWA, and Eastern). Capital merged with United Airlines in 1961.