American Airlines is extending the cancellation of its Boeing 737 Max 8 flights from June until mid-August.
The move is to "provide confidence" to customers during the peak summer travel season, the airline said in a letter.
Last month, 737 Max aircraft were grounded worldwide following fatal crashes involving Ethiopian Airlines and Indonesia's Lion Air.
Boeing is developing new software for the jet's anti-stall system, which has come under scrutiny since the crashes.
Senior officials at American Airlines said they were "confident" that the Boeing upgrade would be approved by US aviation regulators before 19 August, despite its decision to ground its fleet of 24 planes until then.
The cancellations will affect up to 115 flights a day, Reuters news agency reported.
Last week, Southwest Airlines made a similar move and United Airlines has also cancelled 737 Max 8 flights until June.
The plane is grounded as investigations continue into the Ethiopian Airlines crash last month which killed all 157 people on board.
A preliminary report says the aircraft nosedived several times before it hit the ground despite pilots "repeatedly" following the procedures set out by Boeing.
In October a 737 Max 8 flown by Indonesian airline Lion Air crashed, claiming the lives of 189 people.
Investigators are focusing on the plane's anti-stall software, the Manoeuvring Characteristics Augmentation System (MCAS).