It had previously grounded the plane through Feb. 8. The decision comes as the Southwest Airlines flight attendants' union weighs suing Boeing over lost pay as a result of the prolonged 737 Max grounding. The airline's pilots sued Boeing last month over what it said was $100 million in lost wages due to the grounding.
Southwest airlines' fleet is made up of Boeing 737s, and it's had to cancel thousands of flights as a result of the grounding. It is the largest customer of the planes in the U.S. and had 34 in its fleet with another 200 more on order at the time of the grounding in March. Southwest's shares are up approximately 25% year-to-date.
Boeing came under further pressure in late October after documents surfaced detailing that engineers raised concerns about the 737 Max's faulty MCAS flight-control system before two crashes that killed 346 people.
Southwest said it's unable to provide updated first quarter guidance for 2020 due to cancellation extension, the airline said. Southwest's shares were little changed following the decision, while Boeing's shares dipped by about 1% in midday trading.