United Airlines announced that it is adding 270 Boeing and Airbus aircraft to its fleet in a bid to make the airline more sustainable and as it anticipates a resurgence in travel.

In a statement, the Chicago-based airline said this will be the largest order in the carrier's history and biggest by a single carrier in a decade.

Its plan includes addition of 200 Boeing 737 MAX and 70 Airbus A321NEO as well as plans to retrofit 100% of remaining mainline, narrow-body fleet.

When combined with the airline's current order book, United said it expects to add more than 500 new aircraft including about one new plane every three days in 2023 alone.

"Our vision will revolutionize the experience of flying United as we accelerate our business to meet a resurgence in air travel," said United CEO Scott Kirby. 

United noted that by adding these new 737 MAX and Airbus A321NEO aircraft, it means that the airline will be replacing older, smaller mainline jets and at least 200 single-class regional jets with larger aircraft, which the airline expects will lead to significant sustainability benefits compared to older planes.

United expects an 11% overall improvement in fuel efficiency and an expected 17%-20% lower carbon emission per seat compared to older plane models.

"By 2026, United expects its use of aircraft in this new order alone will lower the airline's total carbon emissions per seat up to 15%," United said in a statement.

United has earlier set an ambitious goal to be 100% green by reducing its greenhouse gas emissions 100% by 2050, without relying on traditional carbon offsets.