USAir bought out many other companies in its history, but the "original" USAir was Allegheny.
In 1939, All American Aviation started as a mail service in Western Pennsylvania. They changed their name to Allegheny Airlines in 1953. They used little turboprop airliners until buying thier first jet, a DC-9, in 1966. Allegheny Commuter service, operated by Henson Aviation, began the next year. Allegheny then merged with Lake Central Airlines in 1968 and bought out Mowhawk Airlines in 1972. This made it the sixth largest airline in the world at that time. It changed its name to USAir in 1979. In 1988, PSA was bought out by USAir. In 1989, the fleet of Peidmont was merged into USAir's. It had been acquired by the USAir Group in 1987. Finally, in 1997, USAir became US Airways.
I think more info might be available on the US Airways web site.