Singapore Airlines (SIA) has committed to 20 Boeing 777-9s and 19 787-10, in addition to its previous order for 30 787-10s.
Deliveries of the jets will start as early as in the financial year ending March 2021, the carrier said, adding it also has options to buy six more of each of the two models.
SIA has selected the 777X and additional 787-10s to expand its future widebody fleet.
Singapore Airlines values its latest planned Boeing order at US$13.8 billion based on published list prices.
"We are continuing to invest for the future of the SIA Group".
6 hurt in alleged Palestinian attack in central Israel
Israel, for its part, says 46 Israelis have been killed over the same period in attacks carried out by Palestinians. A wave of street attacks by Palestinians began in October 2015, and had slowed in recent months.
SIA's aircraft commitment to Boeing is an important win for the American firm, which competes aggressively with its European rival Airbus for plane orders.
The order represents something of a reprieve for Boeing after it lost a competition with Airbus in 2015 to provide a widebody capable of flying between Singapore and the USA starting in 2018. There now i 149 orders for the 787-10, which is expected to begin test flights in the coming weeks.
Shares of the airline rose 1.4 percent to S$9.94 on Thursday in Singapore trading before the announcement. "This order is also another demonstration of our commitment to further growing the Singapore hub, as we will be able to offer even more travel options for our customers", says Goh Choon Phong, Singapore Airlines' CEO.
Singapore Airlines now operates a fleet of 85 777s but had yet to order the new 777X variant, which features a redesigned wing, new GE9X engines and a cabin inspired by the 787. Long-haul budget carrier Scoot has orders with Boeing for eight B787s.
The carrier is the launch customer for the 787-10, the first Boeing jetliner to be manufactured exclusively at its new North Charleston, South Carolina, campus.