Singapore Airlines (SIA) on Tuesday formally agreed to previously announced deal with Boeing for the purchase its 39 aircraft worth $13.8 billion during a White House event with Singapore's prime minister.
The airline said in February it would order 20 777-9 and 19 787-10 widebodies as part of plans to modernize its fleet over the next decade.
The agreement was announced in February 2017 as a letter of intent.
The 777-9s are intended primarily for long-haul routes and are due for delivery from the 2021-2022 financial year. The 787-10s are to be operated on medium-range routes and are due for delivery from the 2020/21 financial year, SIA said in a statement.
SIA is due to take delivery of the world's first 787-10 in the first half of 2018, from a 2013 order with Boeing for 30 aircraft.
SIA has more than 50 current-generation Boeing 777 aircraft in service.
SIA's subsidiaries SilkAir, Scoot and SIA Cargo also operate Boeing aircraft.
Shares in Singapore Airlines fell 0.6 percent to S$10.2 on the Singapore Exchange. The stock has gained 6 percent so far this year.