Bombardier Inc., trying to recapture investor confidence after two straight annual losses, is poised to make a major breakthrough with its C Series airliner that would ensure the plane program’s viability into the next decade.

The Montreal-based company, the world’s only manufacturer of both planes and trains, will announce on Thursday that it has won a multibillion-dollar order from U.S. giant Delta Air Lines Inc., according to a person familiar with the situation. The commitment is potentially as large as 125 aircraft, industry analysts have estimated, including 75 planes needed with some urgency and options on 50 more.

Ahead of its annual general meeting Friday morning, Bombardier could also announce the firming up of a letter of intent by Air Canada for up to 75 C Series airliners. That order includes 45 aircraft that Canada’s flagship carrier will definitely take and options on another 30 planes.

“This leaves the C Series program in pretty good shape,” said Ernie Arvai of consultancy AirInsight in Windham, N.H. “Once you begin landing very well-respected customers, you can typically go forward. They’ve got an opportunity to lay out that ‘Yes we’re delivering what we promised way back when and nobody believed.’”

A Delta order would be the largest so far for the C Series program.

Plane makers consider backing from the carrier to be highly valuable, both because of its sheer scale – it is the second-largest airline in the United States by traffic – and also because of its influence. Bombardier’s commercial aircraft sales chief, Colin Bole, calls such customers “market makers.” Many smaller carriers without the same buying analysis resources as their larger peers often wait to see what the big airlines do before making purchase decisions.

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