Space launch company Stratolaunch Systems has rolled its twin-fuselage plane out of its hangar for the first time to conduct fuelling tests.
The company is a unit of Microsoft co-founder Paul Allen's privately owned Vulcan Aerospace.
The aircraft will be the world's largest, with a wingspan of 117 metres (385 ft).
It will serve as an airborne launch pad for putting satellites into orbit. The plan is for the plane to take off from a runway and fly up to 11,000 metres (35,000 ft), the approximate cruising altitude of a commercial airliner. It will then release a rocket carrying the satellite.
The company says it will have the ability to be positioned so satellites can be directly delivered into very precise orbits quickly, without launch range scheduling issues and weather-related delays.
Stratolaunch hopes to perform its first launch demonstration as early as 2019.