Boeing and Southwest Airlines have announced a firm order for 150 fuel-efficient 737 MAX airplanes.

The Dallas-based carrier also ordered 58 Next-Generation 737s in what becomes the largest deal in Boeing history both in dollar value, nearly $19 billion at list prices, and the number of airplanes.

Southwest is the first customer to finalise an order for the 737 MAX and becomes the launch customer for the new-engine variant.

The firm order supports Southwest’s initiative to modernise its all-Boeing fleet, the world’s largest fleet of 737s.

As the 737 MAX launch customer, Southwest will take delivery of the first airplane when it enters service in 2017.

“Southwest is a special Boeing customer and has been a true partner in the evolution of the 737,” said Boeing Commercial Airplanes president and chief executive Jim Albaugh.

“Throughout our 40-year relationship, our two companies have collaborated to launch the 737-300, 737-500 and the Next-Generation 737-700 – affirming the 737 as the world’s preferred single-aisle airplane.

“As launch customer for the 737 MAX, Southwest, Boeing and the 737 continue that legacy.”

The 737 MAX is the new-engine variant of the world’s best-selling airplane and builds on the strengths of today’s Next-Generation 737.

The new-engine variant, powered by CFM International LEAP-1B engines, reduces fuel burn and CO2 emissions by an additional 10-12 percent over today’s most fuel-efficient single-aisle airplane.

It will have the lowest operating costs in the single-aisle segment with a 7 percent advantage over tomorrow’s competition.

“Today’s environment demands that we become more fuel efficient and environmentally friendly, and as the launch customer of the Boeing 737 MAX, we have accomplished both,” said Gary Kelly, Southwest Airlines chairman.

“We are teaming up with our friends from Boeing to lead the industry in a way that makes both our shareholders and our Customers proud to associate with Southwest Airlines.

“Today’s announcement will allow us to maintain our position as a low-cost provider in the years ahead.”

This order continues the growing momentum for the 737 MAX.

With the Southwest order, the 737 MAX has orders and commitments for more than 900 airplanes from 13 customers, while the Next-Generation 737 family has won orders for more than 6,600 airplanes and Boeing has delivered more than 3,800.