United just ordered Airbus' newest jet to replace 50 aging Boeing 757s, while Boeing struggles to develop an alternative

United Airlines signed an order with Airbus on Tuesday for 50 of the French aviation firm's A321XLR aircraft to replace its aging fleet of Boeing 757 jets.

The order was confirmed by United Chief Commercial Officer Andrew Nocella Tuesday evening.

United's order was a major hit from a US airline for Boeing, which has an new midsized airplane, or "NMA," currently in development. The NMA, which is intended to offer a replacement to the 757, with a similar range and capacity, has faced development delays as Boeing scrambles to fix its troubled 737 Max family of jets.

The 737 Max has been grounded worldwide since March 2019, following the second of two fatal crashes within five months.

Although Nocella said that the Boeing NMA had been a contender to replace the 757s, the American plane maker's jet has yet to be announced. On a conference call with media, Nocella said that the airline had not "shut the door on potentially ordering the NMA" in the future, "once Boeing further refines the mission capabilities and details of the NMA."

"But that hasn't happened yet," he added.

This is a developing story, check back for updates.