But, really . . .. How big is it? It’s so big that it has 28 wheels and six 747 jet engines. It’s so big that it has 100km of wire coursing through it.

By comparison - the enormous Airbus A380 has a wingspan of 80 metres. The Stratolaunch’s wingspan is about half as big again, making it the world’s largest plane by wingspan; the famous Spruce Goose, built by tycoon Howard Hughes, had a wingspan of 98 metres.

It’s so big that to truly get a sense of it, you have to see it from a distance - like a mountain.

But why is Allen, the co-founder of Microsoft and owner of the Seattle Seahawks, building such a massive plane?

It’s not to carry passengers, but rather rockets. The bigger the plane, the larger the rockets, or the greater the number.

Allen’s Stratolaunch company has partnered with Orbital ATK to “air launch” the company’s Pegasus XL, a rocket capable of delivering small satellites, weighing as much as 450kg to orbit. The rockets would be tethered to the belly of the giant plane, which would fly them aloft, and once at an altitude of 10,000 metres or so, the rockets would drop and “air launch” to space.

“With airport-style operations and quick turn-around capabilities,” the company said it believes “air launch” is a cheaper and more efficient way to get satellites into space than rockets that launch vertically and can be extraordinarily expensive.

For Allen, it’s all about LEO, or low-Earth orbit. He, and others, such as Richard Branson’s Virgin Orbit, are betting that they can reduce the cost of launching small satellites to space. And that, in turn, will lead to new ways to beam the internet all across the globe, provide better Earth sensing capabilities, better communication, and open up all sorts of avenues.

“When such access to space is routine, innovation will accelerate in ways beyond what we can currently imagine,” Allen said in a statement a year ago. “That’s the thing about new platforms: when they become easily available, convenient and affordable, they attract and enable other visionaries and entrepreneurs to realize more new concepts.”

More than a decade ago, Allen had hoped to spark a revolution in space travel when he funded SpaceShipOne, which became the first commercial vehicle to cross the threshold of space. The project ultimately won the Ansari X Prize, and a US$10 million award. He then licensed the technology to Branson and moved on to other pursuits. But with Stratolaunch, he is back in the space business.

“Thirty years ago, the PC revolution put computing power into the hands of millions and unlocked incalculable human potential,” he wrote. “Twenty years ago, the advent of the Web and the subsequent proliferation of smartphones combined to enable billions of people to surmount the traditional limitations of geography and commerce. Today, expanding access to LEO holds similar revolutionary potential.”

In a statement Wednesday, Jean Floyd, Stratolaunch’s chief executive, said the company would be “actively exploring a broad spectrum of launch vehicles that will enable us to provide more flexibility to customers.”

He added: “Over the coming weeks and months, we’ll be actively conducting ground and flightline testing at the Mojave Air and Space Port. This is a first-of-its-kind aircraft, so we’re going to be diligent throughout testing and continue to prioritize the safety of our pilots, crew and staff. Stratolaunch is on track to perform its first launch demonstration as early as 2019.”

The Antonov Mirya is the world’s largest plane by takeoff weight, at 640 tonnes.