The carrier says the acquisition is the "largest combined order in the airline’s history and the biggest by an individual carrier in the last decade".
The “United Next” plan is expected to increase the total number of available seats per domestic departure by “almost 30%”.
United expects to introduce more than 500 narrow-body aircraft to its fleet in the next four years.
It expects to fly the first 737 MAX 8 this summer and to begin flying the 737 MAX 10 and the Airbus A321neo in early 2023.
Scott Kirby, United Airlines chief executive, said the purchase will "revolutionise" the consumer experience of flying with the airline.
"By adding and upgrading this many aircraft so quickly with our new signature interiors, we’ll combine friendly, helpful service with the best experience in the sky," he added. "We expect the addition of these new aircraft will have a significant economic impact on the communities we serve in terms of job creation, traveller spending and commerce."
