United said it has placed a $900m (pounds 580m) order for 24 Airbus A319 narrow- bodied jets. Airbus beat a rival bid by Boeing with its 737 to secure the deal, which was said to have been signed "at the eleventh hour".
The planes will replace older 737s on US domestic routes as part of United's "retire and replace" programme.
United also confirmed it was placing a $3.5bn (pounds 2.3bn) order with Boeing for 27 wide-bodied aircraft, most of which will be 747-400s.
The deal is a significant boost for Airbus, which has struggled to win orders from US carriers. United broke with tradition in 1992 by ordering 50 Airbus A320s, which are slightly larger than the 319s. Thirty four of these planes have so far been delivered.
An Airbus spokesman said: "It's an important order because we are winning a repeat order in what was seen as Boeing's home market."