The 777X is Boeing's most expensive aircraft - Copyright © 2015 Boeing. All Rights Reserved.

Some of the world’s most prestigious airlines are on tenterhooks as the first flight of an aircraft that could change long-haul travel for decades looms ever closer.

Executives at Singapore Airlines, Emirates and Qatar Airways, among others, will have their eyes cast to the skies this spring when Boeing is expected to fly one of its new 777X planes for the first time.

The 777-9, the first of the X family to be developed, will have the biggest jet engines ever seen, attached to the longest wings of any aircraft ever made by the Seattle-based manufacturer.

The 777X has been said to be the result of the very best of the existing 777 plane, as favoured by the likes of British Airways et al, and the game-changing 787 Dreamliner, which has been praised as one of the most technologically advanced aircraft in history, garnering plaudits from passengers on BA, Norwegian and Virgin Atlantic alike.

It's an “absolute peach”, said Emirates president Tim Clark of the aircraft. The Dubai airline has staked its future on the 777X, ordering 150, the largest single firm order in history. “It is a step change in aircraft design and a step change in propulsion. We are very happy we have got what we wanted,” he told Australian Aviation.

The 777-9 (the smaller sibling, the -8, will follow) is listed as $426 million but will likely sell, considering typical bulk airline discounts, for around $200 million (£155m), making it Boeing’s most expensive plane.

The -9 will be longer than a 747 Credit: boeing

What’s so good about the 777X?

It depends who’s asking. On the one hand, it promises a vast increase in fuel efficiency, working towards an operating cost reduction of up to 18 per cent, which in turn should lead to a fall in fares on long-haul flights. Boeing says it will be the largest and most efficient twin-engine plane on the planet.

On the other, it is another step in the evolution of passenger comfort, with the same benefits showcased on the Dreamliner expected on the 777X, including large, dimmable windows, higher ceilings and an anti-dry, jetlag-beating ventilation system.

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What’s more is its pin-up potential. With a wing-span of up to 71.8 metres and a length of 76.7 metres (longer than a 747), the 777X is a beast, and one that is set to become Boeing’s flagship aircraft.

Dominic Gates, aerospace reporter for the Seattle Times, was part of a press group allowed inside the Everett assembly plant in north-west America ahead of the aircraft’s rollout. “It will be an impressive sight in the sky,” he said. “While most planes look much the same to harried air travellers, early in 2019 Boeing's newest jet may manage to catch and arrest even the casual eye.

“Passengers about to board will see its long, long carbon-fiber wings arc up and away from low on the fuselage, gull-like, then curve downward to the tips. There the wings will end in what will surely be the iconic image of this plane: scythelike wingtips painted with a 777X and folded upward so the jet fits at the airport gate.”

Carrying as many as 414 passengers in a two-class set-up (in the longer 777-9; 349 in three classes), the X is set to become the mainstay of many an international airline.

Can it fly further than any existing plane?

Not quite. Its range is not at the heart of its appeal. The -8 has a projected range of 8,690 nautical miles, and the -9 7,525 nautical miles, both shorter than the 9,700 nautical miles of the A350-900ULR, the aircraft currently serving the world’s longest flight between New York and Singapore.

That said, it has been reported that the 777-8 could serve the “holy grail” of routes, between Sydney and London, carrying perhaps fewer passengers (280) and heading west with favourable winds.

"We think our airplane has the legs and the capability," said Dinesh Keskar, Boeing Senior Vice President Sales Asia-Pacific and India in 2017. "If the 787-9 can do Perth-London, we think that when the 777-8 comes out in the 2021 timeframe we will have a lot more improvement in technology."

It is the Boeing 787 currently being used on the groundbreaking London to Perth route by Qantas. The route’s success makes the likelihood of the X family being put to use on UK-Australia services.

Who will fly it?

Despite the 777 being a stalwart of the British Airways fleet (BA has 58 of the aircraft), the British flag carrier has not yet signalled interest in its younger, shinier sibling, instead placing orders for its Airbus rival, the A350-1000.

But why doesn’t BA want to fly to Australia, too, we hear you cry. It just doesn’t. Willie Walsh, chief executive of IAG, of which BA is a part, said last year: “Code sharing is an option but in terms of using our metal, we're not considering it.

“Personally the idea of sitting on an aircraft for 21 hours to get from Heathrow to Sydney, it does not appeal to me.”

As it stands, seven airlines have orders placed with Boeing for the 777-9, with Emirates boasting the largest. Qatar, Etihad and Lufthansa also have orders placed, while Turkish Airlines has shown willing. Qatar, Emirates, and Etihad are the three to places orders for the -8, too.

Qantas has not yet decided between Airbus and Boeing for its aircraft of choice to forge ahead with plans for “Project Sunrise”, the endeavour to link any city in Australia with anywhere else in the world with a direct flight.

Iran Air previously had $38billion worth of orders placed with Boeing, including 15 777-9s, but these were all but cancelled when President Donald Trump withdrew from the Iran Nuclear Deal in 2016.

The first deliveries of the 777-9 are expected to be made next year.