It converted options it held over the jets into firm orders, with 10 to be delivered each year from 2019-2022, Southwest said in a statement on Thursday announcing its first-quarter earnings.
"This is first and foremost an extension of our fleet modernization strategy," Southwest chief executive officer Gary Kelly told analysts on a post-earnings conference call, according to a transcript.
"We have a very strong business case to replace older 737-700 aircraft given the superior operating economics of the MAX 8."
Southwest is the world's largest operator of 737 jets, with more than 700 of the airplanes.
The airline on Thursday warned the consequences of a mid-air engine explosion on a 737-700 last week will weigh on second-quarter bookings, as investigators probe the events that led to the first passenger death in the airline's history.
Southwest posted first-quarter profit of $438 million, excluding special items, up from $372 million a year earlier.
(Jamie Freed - Reuters)