United Airlines orders 150 Boeing 737s

United Airlines is buying 150 Boeing 737s, planning to use them to replace older planes that are not as fuel efficient.

The order announced on Thursday includes 100 of Boeing’s new 737 Max 9s.That’s a new version of the plane that will have new engines and other tweaks to cut how much fuel it burns. Those planes start arriving at United in 2018.

United is also buying 50 of Boeing’s current 737-900ER. Those planes begin arriving late next year.

The order doesn’t necessarily mean United is getting bigger. The planes that begin arriving late next year will replace older Boeing 757-200s, which are no longer made. It said the new planes are 15 percent more fuel-efficient per seat than the ones they will replace. The Maxes that begin arriving in 2018 could either replace old planes or make United’s fleet bigger, CEO Jeff Smisek said.

The order would be worth more than $14 billion at list prices, although big airlines like United don’t pay list prices.

The 737 is a workhorse of domestic flying. United already flies 43 of those planes, which seat 173 people. For the new 737 Max, United is ordering the biggest version Boeing makes, which seats up to 215 people.

The order is a win for Boeing Co., which has been hoping to boost orders for the Max. Airbus beat Boeing to the punch last year by offering a competing version of its own A320 with a new engine earlier than Boeing did.

Smisek said his airline had “extensive discussions with both Airbus and Boeing” before picking Boeing. He spoke at a news conference in Chicago, where both Boeing and United are based.

Boeing Chairman and CEO Jim McNerney also attended the news conference.

Shares of United Continental Holdings Inc. fell 65 cents, or 2.6 percent, to $24.14 in afternoon trading Thursday, while Boeing shares rose 31 cents to $71.83.