Boeing and China's first privately owned airline, Okay Airways, have finalised a $1.4-billion order for five Dreamliner jets, the two companies said on Thursday.

The purchase of the five 787-9 long-haul aircraft by the Beijing-based airline was signed off in the United States on Wednesday and announced on Thursday via the companies' official accounts on Chinese microblogging site, Weibo.

Okay Airways currently has a fleet of 26 single-corridor Boeing 737 planes, which fly to 70 destinations in China and elsewhere in Asia.

The Chinese company had signed a preliminary agreement to buy the planes back in June, when it has also made a firm order for 15 medium-sized 737 MAX planes at a catalogue price of $1.8 billion.