It is set to be the biggest passenger plane ever seen - with a wingspan so big the tips have to be folded up so it can fit into airports.

The Boeing 777-9X will be the largest and most efficient twin-engine jet in the world, the aerospace giant claims - and will begin production in 2017.

320 have already been ordered, the firm says, and will feature a wider cabin and bigger windows than even the firm's 787 Dreamliner.

The plane will have the widest wingspan of any aircraft: 235 feet, 5 inches (71.8 meters), making it wider than four 53-foot (16-meter) semi-truck trailers parked end to end.

It is so big, Boeing has developed hinges on its wingtips. The hinges will fold up, allowing the plane to shorten its wingspan when it's rolling across airport taxiways, the first design of its kind on any commercial airliner.

The hinged wingtips will measure 12 feet, and locking pins will prevent them from folding during flight, Boeing told CNN.

Inside, the 777-9X will seat at least 400 passengers, 34 more than the 777-9X's competitor - the Airbus A350-1000.

The 777-9X cabin will be 16 inches (40 centimeters) wider than the A350-1000, Boeing says, allowing economy-class seat widths up to 18 inches (46 centimeters).

Typical airline seat widths range from 17 to 18.5 inches (43 to 47 centimeters).

The 777-9X's windows will also be 15% larger than competing airliners, Boeing says, and placed at a height that will be eye level for most passengers.

The factory for the plane, in Everett, is big enough to fit 25 football fields inside.

'Building on the success of the 777 and 787 Dreamliner, the 777X will be the largest and most efficient twin-engine jet in the world, unmatched in every aspect of performance,' Boeing says.

'However, performance is just part of the story.

'The cabin interior of the 777X is inspired by the comforts and conveniences of the 787 Dreamliner and includes larger windows, a wider cabin, new lighting and enhanced architecture — all of which will be custom-tailored for a unique 777X experience.

'The 777X is the future of flight unfolding.

'That's a better way to fly.'