Delta Airlines has placed a 5.6 billion US dollar (£3.84 billion) order for Bombardier's C Series aircraft.
The American aviation giant will buy 75 of the passenger planes - with the possibility of an additional 50. The wings will be manufactured in Belfast.
Deliveries to Atlanta-based Delta are due to begin in 2018.
Alain Bellemare, president and chief executive officer at Bombardier, said: "We are very proud to welcome Delta as a C Series customer and to expand our partnership with such a prestigious airline.
"Given Delta's position as one of the world's largest and most respected airlines, this deal is a strong endorsement of the C Series as the best performing aircraft in the 100-150 passenger class.
"The addition of Delta to our marquee C Series customer list gives us tremendous momentum as we approach entry-into-service this summer."
The innovative carbon-fibre composite technology, developed in Belfast to produce the wings, enables material and aircraft weight savings, contributing significantly to the C Series airliner's stated economic and environmental credentials, Bombardier said.
Delta is taking the smaller CS100 model of the plane.
Bombardier, a Canada-based multinational, has been under financial pressure and invested large sums in its new jet programme, which is crucial to many jobs in Belfast.
In February the firm announced 1,080 job losses at its Northern Ireland plants following delays to the C Series.
Mr Bellemare added: "As a result of this order, the programme is expected to enter into service with a backlog of more than 300 aircraft or up to 800 aircraft including all options and commitments."
The C Series programme received one billion dollars (almost £700 million) from the provincial government in Quebec last year.
Bombardier is one of Northern Ireland's largest employers and supports hundreds of other jobs through its suppliers.
Unions have described the mass redundancies as a cruel blow to a manufacturing sector still reeling from news that JTI Gallahers and Michelin factories in Ballymena are to close.
Michael Ryan, vice-president and general manager at Bombardier Belfast, said: "We're delighted that another customer has placed such a significant order for the C Series aircraft.
"This is a further endorsement of the aircraft's excellent operating economics and environmental credentials, to which we in Belfast are contributing with the production of the advanced composite wings.
"We believe the unique, patented process we've developed represents a step-change in aircraft wing technology, and is strengthening the UK's strategic goal of being the world leader in aircraft wing design and manufacture."
Unite the Union said it hoped the order meant planned redundancies could be avoided.
Regional officer Davy Thompson said: "We hope that today's good news will be followed up by further orders from other airlines.
"The C Series has a huge advantage in terms of fuel efficiency and this contract offers encouragement that other airlines will follow suit."