Paul Allen spent years building the world’s biggest airplane. He’ll never see it fly.

The world’s largest airplane took a stroll the other day. Under the cloudless skies of the Mojave Desert, it rolled out of its hangar and powered down the runway. Slowly at first, then faster, 60 mph, then 70, then 80. But it stopped just short of liftoff, like a sprinter pulling up just before taking the long jump.

The leap to flight will come soon. The plane needs a few more test runs down the tarmac. Then it will fly.

Too bad Paul Allen will never get to see it. The billionaire co-founder of Microsoft, who had quietly been building the monster of an aircraft for several years, died last week without ever seeing his twin-fuselage creation take to the skies.

Which is a shame. Because Stratolaunch was designed not just to make aviation history, but help Allen, and the many others who grew up wanting to make space more accessible to regular people, realize his childhood space dreams.

When Stratolaunch flies it would be the largest plane, as measured by wingspan, ever to take to the skies, bigger even than Howard Hughes' famed Spruce Goose, which flew once, in 1947.

Often better known for his other pursuits, chief among them bringing personal computing software into people's homes, Allen was also an aerospace enthusiast. Stratolaunch was to be his crowning achievement, a massive plane of almost incomprehensible size, capable of carrying as many as three rockets tethered to its belly, which, once aloft, would drop like skydivers, one by one, before launching to orbit from the air.

It had been criticized by some as a vanity project, and Allen as another billionaire with lofty, quixotic ambitions in space.

But as he laid out his plans to me in an interview last year for my book - "The Space Barons," which traces the cosmic dreams of Allen, Elon Musk, Richard Branson and Jeff Bezos - he said he was thinking of ways to harness the power of a revolution in satellite technology for good.

Before we sat down in his Seattle office, which offered a stunning view of the Space Needle and Elliott Bay, his staffers had warned me he might be a touch awkward. He's a genius, they said, but he could be skittish in interviews and as a result didn't do many.

But as he talked about the future he envisioned in space, he was at ease and excited at the possibilities - especially with the advancements in satellite technology.

Just like how computers have drastically reduced in size, while increasing in performance, satellites have gone from the size of school buses to shoe boxes. Being able to put up constellations of them could transform all sorts of industries, he said, and even beam the Internet to all corners of the planet.

"The capabilities of these small satellites is something that's really interesting and fascinating," he said, "both for communications, where a lot of people are putting up constellations of satellites, and for monitoring the challenged health of our planet."

He'd become particularly interested in how space could be used to keep an eye on "things like illegal fishing in the ocean, which is an increasing problem."

The Pentagon was also taking an interest in ways to launch small satellites quickly and affordably. Air Force Secretary Heather Wilson visited Stratolaunch last year. So did Vice President Pence, who heads the National Space Council.

When I met with Allen, we chatted about his childhood passion for space, his first foray into human spaceflight and then most recent plans to build this massive plane, which was so big even he seemed awestruck by its size. That took me by surprise. This, after all, was a man used to doing big things.

On the whiteboard in his office, there were all sorts of projects listed. On the shelf by the window was the Super Bowl trophy his Seattle Seahawks had won. Next to it was a model of his yacht, named Octopus, which cost a reported $200 million to build and came equipped with not one but two helicopter landing pads on its deck and a recording studio once used by Mick Jagger.

But in its ambition and complexity, Stratolaunch dwarfed even that.

"This is big," he told me. "It. Is. Giant."

With six 747 engines and 28 wheels and miles of wiring coursing through its body, the plane is like multiple planes rolled into one - a supersize, Frankenstein-like creation that could only exist in the wild imagination of an eccentric, reclusive billionaire.

In addition to using it to launch satellites, he had another plan, his company revealed to me that day last year: a space shuttle, known as Black Ice that one day could carry people to orbit.

But that was for someday in the future.

For now, they were focused on getting Stratolaunch off the ground.

The stuff of childhood dreams

Allen grew up knowing all the names of the Mercury 7 astronauts as if they were his favorite baseball players, he wrote in his memoir, "Idea Man." And like many kids of his generation, he grew up wanting to become an astronaut. But then in the sixth grade he couldn't see the blackboard at school, he told me in his office, and he knew that meant "my dreams of being an astronaut were over."

His father was the associate director of the University of Washington Library, a second home of sorts for Allen. "My Dad was just letting me loose in the stacks," he recalled. "I loved it."

He devoured not just science fiction, but books about Wernher von Braun, the German-born architect of NASA's might Saturn V rocket, which sent men to the moon. Once, Allen tried to build a homemade rocket of his own using the arm of an aluminum chair packed with powdered zinc and sulfer and firing it from a coffee pot. It didn't work.

He did, however, make it down to the Kennedy Space Center in 1981 to witness the maiden flight of the Space Shuttle.

"The sound was unbelievable," he told me. "The air was vibrating, and you could feel the compression waves going into your chest. You could feel the heat from the engines on your face."

In 2004, Allen made history when SpaceShipOne, the spaceplane he funded, won the $10 million AnsariX Prize challenge by becoming the first nongovernmental vehicle to fly a person to space. In three flights that year, test pilots rode the spaceplane past the 100 km threshold that is generally considered the edge of space. But each flight was risky and harrowing and Allen found himself petrified that someone would get hurt or killed. The risk was just too great.

With computer software "you worst outcome is an error message," he wrote in his memoir. "Now I knew the person whose life hung in the balance, and I found that hard to handle."

But others saw the flights as a precursor to the dawn of a new Space Age, one where corporations would compete and open space to the masses. Richard Branson, the founder of the Virgin Group, was smitten with the idea and acquired the rights to the technology behind SpaceShipOne from Allen.

As they watched the final X Prize flight together, Branson turned to Allen and said, "Paul isn't this better than the best sex you've ever had?"

Allen demurred.

"If I was this anxious during any kind of interpersonal activity, I couldn't enjoy it very much," he wrote in his memoir.

'This is a dream'

Allen went off to pursue other projects, but in 2011 he announced he was getting back in the space game by building Stratolaunch.

"You have a certain number of dreams in your life you want to fulfill," he said at the time. "And this is a dream I'm very excited about."

The announcement came at an interesting time-NASA had just retired the Space Shuttle, leaving it without a way to get astronauts to space. And there was great hope that a new commercial space industry, led by Musk's SpaceX and others, would help open the frontier.

Allen said Stratolaunch would be part of that movement, and keep "America at the forefront of space exploration."

By the time I sat down with him six years later, the giant plane still hadn't flown.

"It's taken longer than expected as almost all rocket projects do," he said.

But he was looking forward to seeing it take off. He couldn’t wait for the day he’d be back out on the flight line in Mojave, watching the plane pick up speed down the runway, moving faster and faster toward a future he had seen in his dreams since childhood.