United Airlines returned to the Boeing narrowbody fold today with an order for 100 of the re-engined 737 MAX and 50 737-900ERs.

United President and CEO Jeffery Smisek says “very extensive discussions” were held with Airbus and Boeing and that negotiations with Pratt & Whitney (P&W) and CFM for the powerplants have been under way for almost a year. The win reflects “what we believe to be the best airplane with the best engine with the best price,” he notes.

United, which was not a customer of the 737 Next Generation family (which includes -700s, -800s and -900s) and flies A319s and A320s, merged with Continental Airlines, a major 737 NG customer with no Airbus aircraft, in 2010.

The win for Boeing indicates that United’s domestic fleet favors the Seattle airframer’s portfolio. It also was a blow for P&W’s PW1100G engine, which could have been selected had United opted for the A320NEO. CFM holds exclusive rights to supply the 737 NG and MAX series with its CFM56-7B and Leap-1B engines, respectively.

United currently operates 152 A319s and A320s, some of which will be displaced by the incoming 737-900ERs. The Continental fleet, meanwhile, counts as Boeing’s largest domestic customer with 332 NG orders, including 47 737 NGs still to be delivered—38 of the smaller 737-700s and nine 737-900ERs.

Despite this shift, United’s Airbus single-aisle fleet will be part of the merged airline’s fleet “for years to come,” says Smisek.

The CEO also says the Boeing order includes options to be announced later in a U.S. Securities and Exchange Commission filing.

United chose the largest 737s available both in the current NG family and the new MAX, which is scheduled to enter service in 2017. In doing so, it underscores a trend across the airline industry toward greater seat counts in single-aisle transports. As Boeing’s replacement for United’s 155 757-200s, the 737-900ER is sized for as many as 220 passengers and the 737-9 for 215.

While the 737 win places United strongly in Boeing’s camp, Smisek notes that the carrier continues to split its interests between Airbus and Boeing widebodies. It has 25 A350-9s on order and in September will receive the first of 50 787s.

The list price of $14.7 billion includes $5 billion for CFM56-7B and Leap-1B engines and an engine service agreement valued at $5 billion; however, major customers typically pay as little as half the list prices.