Stratolaunch has a wingspan longer than a football field and comes equipped with two cockpits, 28 wheels and six engines normally used to power 747 jumbo jets.
It will be used to transport rockets carrying satellites and other objects into the Earth's upper atmosphere, where they will blast off into space.
One such payload, Black Ice, will be designed to last at least three days in the harsh environment of space and will be around the same size as the Space Shuttle used by Nasa until 2011.
It could run supplies to the International Space Station, deliver science experiments into orbit and - once it has been deemed safe enough - even carry astronauts into space.