Calgary-based WestJet Airlines Ltd. has announced it will expand its fleet with the addition of 65 Boeing 737 MAX aircraft.

The carrier announced Friday that it has signed a letter of intent to buy 40 737 MAX 8 and 25 737 MAX 7 aircraft, with delivery scheduled to begin in September 2017.

WestJet's current fleet includes 103 Boeing Next-Generation 737 and four Bombardier Q400 NextGen aircraft.

The airline has changed an earlier order from Boeing and will substitute 15 Boeing Next-Generation 737 aircraft scheduled for delivery between December 2014 and 2018 with Boeing 737 MAX aircraft. It previously announced a deal for 10 737 Boeing Next-Generation aircraft.

The order marks an ambitious expansion to the fleet as WestJet, which launched its regional carrier, WestJet Encore, in June takes on new routes. The airline reported an 8 per cent annual increase in passenger traffic last month.

The 737 MAX aircraft are the newest incarnations of the hard-working 737. Boeing says the 737 MAX aircraft is the most fuel-efficient single-aisle aircraft on the market as its CFM International LEAP-1B engines have lower than average carbon dioxide emissions.

WestJet now anticipates its capital expenditures of $210 million to $220 million for the third quarter of 2013, and between $690 million and $710 million for the full-year 2013. Previously, WestJet had anticipated between $100 million and $110 million for the third quarter of 2013, and between $610 million and $630 million for the full-year 2013.

A final purchase agreement with Boeing is expected before the end of September.