Stratolaunch, which is the world's largest airplane and has flown only once, is up for sale.

Holding company Vulcan is seeking to sell Stratolaunch for $400 million, people familiar with the matter told CNBC. Vulcan is the investment conglomerate of late billionaire Paul Allen, a Microsoft co-founder. Allen died last October following complications of non-Hodgkin's lymphoma.

The hefty price tag includes ownership of the airplane as well as the intellectual property and facilities.

Stratolaunch is the world's largest airplane by wingspan, which stretches 385 feet — longer than an American football field. The plane is powered by six jet engines salvaged from Boeing 747 aircraft.

Allen's vision of a massive plane that can launch rockets from the air was at least partially fulfilled in April, when Stratolaunch flew for the first time after about eight years in development. Based at the Mojave Air and Space Port in California, the giant airplane flew for more than two hours before landing after what was deemed a successful first flight.

Stratolaunch Systems has had various partnerships, as well as internal plans, for the rockets the Stratolaunch aircraft will carry. SpaceX was one of the company's earliest partners, but Stratolaunch later switched to a contract with Northrop Grumman-owned Orbital ATK to fly the Pegasus XL rocket. Stratolaunch's plan to develop its own fleet of rockets was scrapped in January.

While neither Stratolaunch nor Vulcan would comment on the sale discussion, a company representative told CNBC that "Stratolaunch remains operational. We will provide an update when there is news to share."

Stratolaunch has been steadily downsizing this year, with much of the workforce laid off already, despite plans to launch a small Northrop rocket in 2020. The company is in the process of closing operations, Reuters reported last month.

One item holding up the sale of Stratolaunch, according to one of the people, is an internal disagreement between CEO Jean Floyd and Paul Allen's sister Jody Allen, who serves as the chair of Vulcan as well as the executor of his estates. While Floyd appears to be petitioning that Vulcan keep the Stratolaunch program alive, especially by retaining the company's intellectual property, Jody Allen would like to sell the company outright, the person said.

There are number of possible suitors for Stratolaunch, especially the most active space industry trio of billionaires: Elon Musk, Jeff Bezos and Richard Branson.

While it's unknown if either Musk's SpaceX or Bezos' Blue Origin is pursuing a purchase of Stratolaunch, people familiar told CNBC that the company has spoken to Branson about selling to his Virgin Group. Branson's conglomerate owns three space companies: Virgin Galactic, The SpaceShip Company and Virgin Orbit. As the former two were built using technology similar to that of Stratolaunch — all three have a similar manufacturing heritage, as they all originated from designs by prototype aerospace manufacturer Scaled Composites — Virgin may be an ideal destination for Stratolaunch.

But Branson was hesitant to pay full price for Stratolaunch, a person familiar said. Instead, the person said, Branson countered Vulcan's offer in a way similar to his lowball offer for the supersonic Concorde fleet in 2003. British Airways had been planning to retire the airplanes, and Branson had offered the airline $8.30 to take the planes off its hands and continue flying the Concordes for Virgin Atlantic.

Branson pitched Vulcan that he would buy Stratolaunch, the person said, for $1.