Singapore Airlines’ regional carrier SilkAir has signed a letter of intent to buy new aircraft from Boeing worth $4.9 billion, making it the largest order in SilkAir’s history.

The agreement comprises firm orders for 31 Boeing 737 MAX and 23 737-800s, as well as purchase rights for another 14 aircraft, SilkAir said in a statement.

The aircraft are scheduled for delivery between 2014 and 2021, the airline said. SilkAir currently operates 21 A320s.