American Airlines is extending the cancellation of its Boeing 737 Max 8 flights from June until mid-August.
The airline said in a letter the move is to provide confidence to customers during the peak summer travel season.
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 August 19, despite its decision to ground its fleet of 24 planes until then.
Reuters news agency reported the cancellations will affect up to 115 flights a day.