With a deal for 150 firm Boeing 737 MAX narrowbodies and options for 150 more, Southwest Airlines has become the launch customer for the aircraft type.

Boeing says it has more than 700 commitments for the aircraft and that nine customers plan to acquire the rival to the Airbus A320NEO. American Airlines was the first to announce plans to take the 737 MAX, but Southwest’s deal is the first firm order for the type.

The airline says it will receive the first aircraft in 2017, with deliveries of the firm order portion due to stretch to 2022. The airline has the flexibility to take either -7s or -8s.

Moreover, Southwest announced it is using the deal to buy additional 737NG capacity in the interim, taking 58 additional NGs, including 25 converted options. The order book also includes 78 options.

Southwest also says it has substituted -800s for all -700 737NG deliveries scheduled for 2012 and 2013 in addition to a portion of its 2014 deliveries.

The total Southwest 737 firm order book now stands at 350 units.

In announcing the deal, Mike Van de Ven, Southwest Airlines Executive Vice President and Chief Operating Officer said, “We are enthusiastic about our fleet modernization plans, and especially about becoming the launch customer for the Boeing 737 MAX.” He added that “the much improved fuel efficiency of the 737 MAX will enable us to improve our fuel costs, as well as our environmental performance, with great comfort and reliability to offer to our customers.”