The new timelines mean that the three carriers will be flying without the 737 MAX for the second straight US summer, an issue that hit their profits during last year's peak travel season.
United said Friday that it has taken the Max out of its schedule until at least September 4.
United said it took the plane out of its schedule until at least September 4.
Boeing CEO David Calhoun discusses restarting the production of 737 Max jets. The latest unofficial guidance was for the flight to happen this month, but now it is unlikely to occur while Boeing tries to resolve the new problems, sources said this week.
American Airlines, which had 24 Max planes at the grounding, still has the plane scheduled to return in early June, but company officials have also said they will likely have to extend it.
United, Southwest and American were the only USA airlines operating the Max when it was grounded.
By taking the planes out of the schedule now, the airlines hope to avoid last-minute cancellations that could leave passengers stranded.
The airlines are bracing for a second straight summer without the jet, which was expected to become a bigger part of their fleets.
Boeing's 737 MAX was grounded worldwide last March after two crashes in Indonesia and Ethiopia that killed 346 people within five months. According to Ethiopian Airlines, contact with the plane was lost at 8:44 a.m., just a few minutes after it took off.
Boeing is testing updates to flight-control software that played a role in the crashes by pushing the planes' noses down based on faulty readings from sensors.
Although Boeing has claimed that the US Federal Aviation Administration (FAA) would recertify the plane in mid-2020, the regulatory body has yet to outline a clear timeline for regulation.