United Continental is buying 150 new generational short haul Boeing aircraft is a deal worth $14.7 billion (£9.5 billion).
United will purchase 100 Boeing 737 MAX 9 aircraft and 50 Boeing 737-900ERs for delivery between 2013 and 2022.
The carrier become the North American launch customer for the 737 MAX 9 which is due to enter service from 2018.
The new 737s will replace older, less-efficient aircraft to reduce fuel and operating costs. The 737 MAX 9 is expected to achieve fuel burn and CO2 emission reductions of up to 13% compared to current 737 aircraft.
Larger overhead bins will allow the aircraft to accommodate additional carry-on bags.
United said the deal solidifies its order book with 272 new aircraft deliveries due through to 2022, including 50 Boeing 787 Dreamliners and 25 Airbus A350XWBs. United will be the first North American carrier to take delivery of the 787 in September.
President and CEO Jeff Smisek said: “This order is a major step in building the world’s leading airline, and we look forward to offering our customers the modern features and reliability of new Boeing airplanes, while also making our fleet more fuel efficient and environmentally friendly.
“New aircraft deliveries support our flexible fleet plan, permitting us to tailor future capacity up or down, based on changes in demand or other market conditions.
“These new aircraft, combined with our new widebody aircraft already on order and our existing fuel-efficient aircraft, will solidify United’s future fleet as one of the most efficient and environmentally responsible fleets among our global competitors.”
United’s 50 additional 737-900ERs to be delivered from late next year will mainly be used to replace older, less-efficient Boeing 757-200s on US domestic routes. The new aircraft are expected to burn up to 15% less fuel per seat than the those they are replacing.
United was the North American launch customer for the 737-900ER in 2006 and currently operates a fleet of 43 aircraft.