Singapore Airlines’ (SIA) narrowbody operator SilkAir has signed a letter of intent (LOI) to purchase Boeing 737-800s and 737-8s, dealing a major blow to Airbus.

SilkAir currently operates a fleet of 21 A319s/A320s, and still has three A320s on order for delivery by the end of next year. The carrier says it has signed a LOI for 23 737-800s and 31 737-8s, with deliveries in 2014-2021. There are also purchase rights for an additional 14 aircraft. The firm order for 54 aircraft will more than double its fleet and replace its Airbus narrowbodies, SilkAir says.

The 737-800 is larger than the A319, but SilkAir says under the deal being negotiated with Boeing it will have the flexibility to switch some orders to other 737 variants.

The LOI is not legally binding, but the airline plans to sign a firm contract within a few months, a SilkAir spokeswoman says. When asked why SilkAir chose the 737-800 and 737-8 over the A320 and A320NEO, the spokeswoman replied: “We decided based on the proposals. Boeing was a better fit and met our objectives.”

Besides Airbus and Boeing aircraft, SilkAir also considered the Bombardier CSeries and Embraer E-190. “For now, we have no definite plan to order these small aircraft,” the spokeswoman says.

Introducing the 737 into its fleet will be a massive task for SilkAir. The carrier will have to invest in new training equipment and retrain its pilots.

SilkAir has been operating Airbus A320-family aircraft since 1998. Before that, it flew 737-300/400s.