United Continental Holdings Inc. (NYSE: UAL) placed orders for 150 single-aisle Boeing (NYSE: BA) 737 airplanes for $14.7 billion, the Chicago-based airline said Thursday.

United ordered 100 737 Max 9 jets and 50 737-900ER models. The 100 737 Max 9 is the priciest and most sophisticated version of Boeing's refurbished narrow-body jet. The planes will be delivered from 2013 through 2022, United said.

The Max 9 jets are expected to be 13 percent more fuel-efficient than the carrier's current 737 aircraft model. United will start receiving the Max 9 mdoels in 2018 while the 737-900ERs will arrive the carrier's flight test facility next year.

Customer demand for the 737 family of airplanes is growing unabated, Boeing said. Over the past week alone, customers announced commitments for 396 airplanes valued at more than $37 billion. The 737 Max has over 649 orders in its books to date.

Boeing says this order propels the 737, the most successful jetliner in history, beyond 10,000. The 737 program now stands at 10,039 orders plus deliveries. No aircraft has sold more. The first model, the 100 series, had its first flight 45 years ago in 1967. To compare: that plane was a 100-seater that could fly nonstop at most 1,800 miles.Successive stretches and later models brought capacity to 200 seats with a maximum range of more than 6,000 miles.

The Airbus A320 and A320neo family is in second place with more than 9,000 between delivered and ordered.

United's $14.7 billion order includes 100 of the new 737 MAX 9, the biggest member of the jetliner family, which has not flown yet and has in fact not begun production; and 50 of the current 737-900ER model. Deliveries will be from 2013 for the 900ERs and 2018 for the MAX 9s, and end in 2022. They will replace the Boeing 757s United currently uses on longer-range domestic routes and are, according to Boeing, 15 percent more fuel-efficient than the 757s in terms of fuel burned per seat-mile flown.

The 737 MAX series, expected to begin commercial service in 2017, now has 1,200 orders from 18 customers, according to Boeing.

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