Delta Air Lines has signed a firm agreement to buy 75 CS100s and take options on another 50 of the 110-seat aircraft in a major victory for Bombardier Commercial Aircraft.

The order valued at $5.6 billion includes conversion rights and Delta “may may elect to convert a number of these aircraft into CS300 at a later date”, Bombardier says.

“As we reshape our fleet for the future, the innovative onboard experience of the C Series is a perfect complement for the top-notch service provided every day by Delta people,” said Ed Bastian, Delta’s incoming chief executive.

Delta is now the largest customer for the Bombardier twin-jet, powered by Pratt & Whitney PW1500G engines. Fred Cromer, president of Bombardier Commercial Aircraft, calls the deal a “watershed” moment in the programme’s history.

“This order is a resounding endorsement of the CS100 aircraft performance and its exceptionally low operating cost,” Cromer says.

Bombardier had 250 firm orders on backlog for the CSeries before the deal was announced. It was not immediately clear if the order would be added to the backlog today or sometime in the future.