Southwest Airlines Co. is getting new, larger planes that could pave the way for the all-domestic airline to offer international flights.

Chief executive officer Gary Kelly said Wednesday that Southwest would change an order with Boeing Co. and get 20 new 737-800 jets beginning in March 2012.

Those jets have 175 seats, 38 more than the largest planes in Southwest's current fleet, which means potentially more revenue per flight.

The airline plans to add international service when it completes the purchase of AirTran Holdings Inc., which flies to Mexico and the Caribbean. It could still use AirTran's planes for those flights.

Kelly said during a speech in New York that Southwest might use the 737-800 first at busy airports in the Northeast. But he said it could also be used to fly to Hawaii, Alaska, Canada, Mexico, and the Caribbean - destinations that Southwest does not serve now.

Southwest notified Boeing it would substitute 737-800s orders for ones it had already placed for the 737-700. Kelly said Southwest would retire older planes as it gets the new ones. He also said Southwest did not plan to expand its fleet.

The airline did not disclose financial terms of the switch, but the bigger plane is more costly. Boeing lists the average price of a 737-800 at $80.8 million, compared with $67.9 million for the 737-700. However, aircraft manufacturers routinely give discounts to good customers.

Southwest operates more than 400 planes, all various models of the one-aisle 737. It is a short- to medium-range jet that Boeing began producing in the late 1960s and has updated many times since.

For years, the reliance on one type of plane has been seen as a strength at Southwest because it kept things simple. It was cheaper to train pilots and mechanics to work on just one type of plane.

Southwest's union pilots and flight attendants approved of buying the larger planes. It could mean more jobs because an extra cabin attendant would be required on flights using the 737-800.

Southwest, based in Dallas, carries more U.S. passengers than any other airline, although it ranks behind the combined United and Continental, as well as Delta and American in traffic, which all operate international flights.