"This order is a major step in building the world's leading airline, and we look forward to offering our customers the modern features and reliability of new Boeing airplanes, while also making our fleet more fuel efficient and environmentally friendly," Jeff Smisek, United's CEO, said at a press conference in Chicago, where both United and Boeing are headquartered.
United will begin receiving 737-900ERs next year. Deliveries of the MAX, which is in development, begin in 2018. The 737 is assembled in Renton.
The order comes as Boeing and rival Airbus are wrapping up the Farnborough International Airshow outside London. Airbus said Thursday that it had won orders valued at $11.1 billion and commitments valued at $5.8 billion.
The United deal solidifies ties between Boeing and United, which has a very large fleet of 697 Boeing and Airbus planes. "Boeing and United grew up together," said Boeing CEO Jim McNerney. "We were part of the same company at one point."
The order also builds on Boeing's growing success with U.S. carriers. Boeing scored a 100-plane purchase from Delta Air Lines last August. Last July, Boeing and Airbus split a 460-jet order with American Airlines, before Boeing had launched the re-engined 737 MAX. Airbus had already announced development of the A320neo, a re-engined version of the European company's hot-selling single-aisle model.
United will be the North American launch customer for the 737 MAX 9, which now has more than 1,200 orders and commitments from 18 customers. The MAX series will be the fourth generation of the 737. While it will resemble previous models, it will have more efficient engines and other improvements designed to rival the A320neo.
The order is United's first since a 2010 merger creating the carrier from former United parent UAL Corp. and Continental Airlines. Boeing was Continental's exclusive plane supplier for two decades, but United had most recently bought its narrowbody planes from Airbus. Smisek noted that United has A350s on order with Airbus and will continue to have Airbus jets in the fleet for "a long time to come."
Overall today, Boeing planes comprise about 78 percent of United's mainline jets, with the rest made by Airbus. A 555-plane regional fleet is split about evenly between Bombardier and Embraer.
United already flies 234 737s, including a few -900s, the largest of the current line. The new 737 MAX 9 will replace the -900.
The order doesn't necessarily mean United is getting bigger. The planes that begin arriving late next year will gradually replace 155 Boeing 757-200s, which are no longer made, United's Smisek said.
Smisek said the airline had been seeking to buy single-aisle aircraft for the past year, talking both with Boeing and Airbus as well as with engine-makers.
"We negotiated what we believe to be the best airplanes with the best engines at the best price," he said.
United's MAX jets will come with LEAP-1B engines from CFM International.
United also is expecting to receive its first Boeing 787 this year. The carrier has 50 on order.
"Boeing has guaranteed us -- I'm looking at Jim McNerney -- that we'll get our first 787 in late September," Smisek said.
Boeing is trying to reclaim the top spot in commercial production lost to Airbus in 2003. Toulouse, France- based Airbus had record orders of 1,419 aircraft in 2011, while Boeing's tally was 805. Airbus announced 95 percent of narrowbody orders at the Paris show last summer but has said 2012 orders may fall by half as an initial flurry of A320neo purchases wanes.
Boeing said the United order puts requests for the 737 family over the 10,000 mark. The plane was first built in the 1960s.
"To witness the first commercial airplane to surpass 10,000 orders is monumental," said Beverly Wyse, vice president and general manager of the 737 program.
The Associated Press contributed to this story.