United Airlines started selling tickets for its first 787 flights on Tuesday, saying it plans to start service between Denver and Tokyo on March 31.

As of Tuesday those will be the first commercial 787 flights on a U.S. airline. Boeing delivered the first 787 late last year to Japan's All Nippon Airways. Airlines have been eager for the fuel-efficient, midsized plane.

At one time, Continental, which merged with United in 2010, had planned to introduce 787 flying from Houston to Auckland, New Zealand. Its 2010 merger with United Airlines put United at the front of the line for U.S. airlines waiting for that plane.

So, which will be first? Denver-to-Tokyo or Houston-to-Auckland? The answer may be tied up with United's tussle with Southwest Airlines Co. over international flying out of Houston.

United's biggest hub is at George Bush Intercontinental Airport in Houston. United has been tussling with Southwest Airlines Co. over Southwest's effort to win permission for international flights from Houston's William P. Hobby airport. United has opposed the addition of international flights at Hobby.

On Tuesday, United spokesman Christen David said the airline had planned Houston-Auckland flights "on the assumption that (Bush Intercontinental) would be the sole international airport, maximizing connecting traffic. If that isn't the case, the flight won't work."

Shares of Chicago-based United Continental Holdings Inc. fell 9 cents to close at $21.83.