Shares of Southwest Airlines were up slightly in premarket trading to $55.22, while Boeing rose modestly to $330.84.
The Dallas airline, the largest operator of MAX airplanes worldwide, had previously canceled flights through April 13.
The plan would cut about 330 weekday flights from Southwest's total peak-day schedule of more than 4,000 daily flights, Reuters reported.
On Tuesday American Airlines (AAL) - Get Report extended cancellations of the 737 MAX through June 3, after saying earlier it would resume flights with a limited number of the planes on April 7. United Airlines (UAL) - Get Report was the first to announce the cancellations in December.
The 737 MAX has been grounded following two deadly crashes in five months that killed 346 people. Airlines that own the plane are cancelling more than 10,000 monthly flights.
On Tuesday, Boeing reported its final sales numbers for the year, which showed that the plane maker in 2019 delivered less than half the number of planes that it did in 2018.
The company delivered 380 commercial planes in 2019, including 79 in the fourth quarter, after delivering 806 planes a year earlier.
Earlier this month, the Federal Aviation Administration said Boeing was facing a $5.4 million penalty for installing faulty parts on 178 of its 737 MAX aircraft.
The FAA charged that “Boeing failed to adequately oversee its suppliers to ensure they complied with the company’s quality assurance system.”
Last month, the FAA proposed a $3.9 million penalty for Boeing for installing the same kind of faulty parts on 133 of its 737 NG airplanes.