Last month Microsoft billionaire and philanthropist Paul Allen announced that he’s started a new space project called Stratolaunch. It’s a follow-on to his SpaceShipOne project that became the first privately built craft to send people out of the atmosphere in 2004.

With him at the press conference was SpaceShipOne designer Burt Rutan, fueling press coverage that the dream team of Allen and Rutan had once again teamed up for an innovative space launch project. That’s only partly true; Rutan is retired now, and while he’s serving on the Stratolaunch board as an advisor, it will be engineers at his former company, Scaled Composites, who design the launch aircraft. SpaceX will design and build the rocket to be launched by the aircraft. Huntsville-based Dynetics will build a connecting system to mate the two vehicles.

I spoke with Stratolaunch CEO Gary Wentz, a former NASA engineer and manager, to get further details on the company’s plans and where it is now in development. According to Wentz:

-Scaled, SpaceX, and Dynetics are acting as contractors to Stratolaunch, not partners. The effort is funded entirely by Stratolaunch, through Paul Allen’s Vulcan Inc.

-Through Scaled, Stratolaunch has purchased two used Boeing 747s from United Airlines. The airplanes will be flown into Mojave Air and Space Port early next month for Scaled to take them apart and use their components in building the world’s largest aircraft, the 385-foot-wingspan Stratolaunch mothership.

-The mothership is currently known only by its Scaled model number: M351.

-The Stratolaunch plan calls for the mothership to carry a to-be-built SpaceX Falcon 4 rocket to high altitude from which to launch a payload to orbit. The Falcon 4 will be powered by 4 SpaceX Merlin IB engines (contrary to the now-out-of-date Stratolaunch video, which shows 5 engines).

-Design on the mothership is planned for completion by late summer of next year.

-The mothership is to begin flight testing in late 2015 in Mojave, with rocket test launches from the airplane to begin at Cape Canaveral in late 2016.

-First revenue flights will be unmanned payloads; manned flights, Allen’s ultimate goal, could begin around 2020.

-Stratolaunch has begun clearing dirt at the Mojave Air/Space Port for a 20-acre site to house the aircraft and assembly facilities in two buildings, which are to be completed in the next 18 months.

-The retired Space Shuttle processing buildings and runway at Cape Canaveral are envisioned as Stratolaunch’s base of operations.

-Stratolaunch is not actively seeking customers yet, just focusing on development right now.

Of his motivation for leaving NASA after 18 years to work on Stratoaunch, says Wentz:

“My whole history with NASA was flying flight hardware, and I just saw this as the next opportunity to do that knowing that with the cancellation of the Constellation program it would be 5 to 8 years before they’re in a mode to be able to fly more hardware at the level at which I had become accustomed. It was a good opportunity for me to come out and build something different and actually fly some hardware.”