Boeing employees cheer as the first  787 Dreamliner takes off from the runway at Paine Field in Everett, Wash. in December. (Mike Siegel/Seattle Times/AP)

By Julie Johnsson | UAL Corp.’s United Airlines said Thursday it had finalized its order to buy 25 of Boeing Co.’s 787 Dreamliners with an option to buy another 50 jets, one half of an order that’s expected to total $8 billion.

The Chicago-based carrier, however, still hasn’t finalized terms with Airbus SAS to purchase 25 A350-XWB aircraft, with options for another 50 jets. United said in a Securities and Exchange Commission filing Thursday that it expects to close that deal by the end of March.

Until it announced the deal with Boeing and Airbus in December, United hadn’t ordered new aircraft for 11 years and had last planned a major make-over of its fleet in the early 1990s.

It was the only large U.S. carrier to remain on the sidelines as global carriers went on a aircraft buying spree during the last decade.

But United waited until 2009 to act, taking advantage of a depressed aerospace market that left Boeing and Airbus scrambling for orders. United, the third-largest U.S. carrier, is expected order narrow-body jets this year and has said it will solicit offers from regional jet-makers Embraer and Bombardier in addition to Boeing and Airbus.

United doesn’t plan to begin adding the wide-body planes to its fleet until 2016, which should help the carrier avoid the production delays that made fleet-planning a nightmare for early Dreamliner customers.

Obtaining large numbers of options also gives United the flexibility to shift plans if one of the planes fails to deliver on the range or fuel-efficiency promised by manufacturers, a risk with game-changing jets like the 787 or A350, analysts said.

“There’s an awful lot of fudge in this,” said Hubert Horan, a Phoenix-based aviation consultant who formerly led international network planning for Northwest Airlines. “If the 787 program totally collapses, they can go to Airbus and say, ‘We want those planes now,’ or vice versa.”

United’s upfront costs should be relatively light, according to the SEC filing. United expects to pay $700 million through 2014 to secure its place on the manufacturers’ assembly lines, and the remaining $7.3 billion after that point.

The Boeing order is valued at $4.2 billion at average list prices, but airlines usually receive discounts for large orders. The planes should be delivered as United retires the Boeing 747 and 767 models that it uses extensively for long-range international flights.

The Dreamliner made its first flight in December, more than two years behind schedule, and is slated to be delivered to launch customer All Nippon Airways later this year. Airbus hasn’t yet begun to assemble the A350. Its first delivery is slated for 2013.

Dow Jones Newswires contributed