Unlike some of its largest domestic rivals, United already flies both Airbus and Boeing planes, giving both manufacturers an incentive to try to grab a bigger share of a major airline's business. As part of its order, United is hoping to simplify its fleet by ending up with fewer different types of aircraft, a change that would cut its maintenance and crew-training costs.

United could sign a major order as early as the fall, the people familiar with the matter said, if Boeing or Airbus agree to certain conditions. The most crucial would be financing arranged by the manufacturer that doesn't eat into United's cash, these people said. United also wants the flexibility to change the order later, according to these people.