The planes have been grounded globally since mid-March following two crashes that killed 346 people. The grounding has dragged on much beyond initial expectations as Boeing has upgraded systems and faced questions from regulators and politicians over the plane.
Domestic-focused Southwest, which had previously set February 8 as a return date for the MAX, said "proactively" removing the aircraft from service reduced the likelihood of last-minute cancellations and disruption.
"Southwest Airlines continues to monitor information from Boeing and the Federal Aviation Administration (FAA) on the impending 737 MAX software enhancements and training requirements," Southwest said. "We remain confident that, once certified by the FAA, the enhancements will support the safe operation of the MAX."
A few hours later, American announced a similar move after previously grounding the plane through January 15. American said adjusted the timeframe followed "continuous contact" with the Federal Aviation Administration and Boeing.