While the plane could make Europe from Australia's east coast or New York from Auckland, it is crucial there are no payload penalties. Airlines want to be able to fly full planes to make them profitable.
The 777X will compete against the Airbus A350-900ULR, an aircraft that will beat it into service. From next year, Singapore Airlines will fly the ultralong-range A350 more than 15,000km from its base to New York.
Both Air New Zealand and Qantas will assess the Boeing and the Airbus aircraft for their fleet replacement programmes next decade.
Dinesh Keskar, senior vice-president, Asia Pacific and India Sales, Boeing Commercial Airplanes, said the 777X programme was on schedule for a test flight in 2019 and delivery to its launch customer, Emirates the following year.
The 777-9 version can carry up to 425 passengers with a range of 14,075km, while the 777-8 seats up to 375 passengers with a range capability of 16,110km - almost 2000km further than the distance from Auckland to New York.
Keskar told the Herald Air New Zealand had requested information about the new 777s.
"We work with all airlines around the world to keep showing them what are the new technologies," he said.
"Having worked with these airlines for a number of years, we understand what their aspirations are and what their future requirements are."
The wingspan of the 777X is 72m and can fold up about 6m from the wingtips.
Keskar said this would allow the aircraft to use gates at airports that another Airbus rival, the A380, cannot use as it has a fixed wingspan of 80m.
Folding wings have been used for decades, notably on aircraft carrier-based planes, but Boeing had simplified the technology for the 777X. Rather than hydraulic mechanisms, the new plane would have a wire running out to a small motor at the point of the fold.
"By doing so there's no complexity - a lot of pilots have come and had a look at it. They're very comfortable with it."
Boeing had opted for the traditional aluminium fuselage rather than carbon fibre because testing had revealed it was as strong as it needed to be for larger windows, and to be pressurised to a lower altitude than existing 777s, which first flew in 1994.
The Boeing Dreamliner fuselage is made from a large proportion of carbon fibre, but to do the same with the new 777 would have required an investment in new autoclaves and pushed up costs that airlines could have baulked at.
"It would have outweighed the benefit of doing that - so that's the trade," said Keskar.
Boeing this week announced 20 orders for the 777X from Singapore Airlines. So far there have been 326 orders for the plane, including from all three Gulf carriers.
The new 777
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The 777-9 can carry up to 425 passengers as far as 14,075km
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The 777-8 seats up to 375 passengers as far as 16,110km
Grant Bradley travelled to Seattle courtesy of Qantas and Boeing
