The first addition to the Boeing 787 family took off on Tuesday from Paine Field in Everett, Washington State near the factory where the plane was assembled, to the cheers of a couple of hundred Boeing employees
The 787-9 is 20 feet (six metres) longer and can seat 40 more passengers than the original 787-8, which carries between 210 and 250 passengers.
The new version of the fuel-efficient, long-haul wide-body known as the Dreamliner also can carry more cargo and fly further, according to Boeing.
After flight tests and certification, the first 787-9 will be delivered next June to the launch customer, Air New Zealand.
The original 787-8 was delivered in September 2011, nearly three years late because of production problems.
The worldwide fleet of about 50 planes was grounded for almost four months this year after lithium batteries smouldered on two planes in January.
The redesigned battery system, which resolves the overheating problems, is built into the 787-9.
Boeing plans another stretch with the 787-10. That plane would seat between 300 and 330 passengers.