The world's largest aircraft by wingspan has successfully completed its first flight, the US space transportation firm behind it says.
The plane built by Stratolaunch has two fuselages and a wingspan of 117 metres.
It flew for 2.5 hours over California's Mohave Desert.
"What a fantastic first flight," Stratolaunch's chief executive Jean Floyd said in a statement on Saturday.
"Today's flight furthers our mission to provide a flexible alternative to ground launched systems."
Stratolaunch was established in 2011 by late Microsoft co-founder Paul Allen. It aims to make access to orbit more affordable and accessible by launching satellites into space from aircraft, rather than from the ground.
On Saturday its aircraft took off from the Mohave Air and Space Port, about 150 kilometres north of Los Angeles. It flew at altitudes up to five kilometres and performed numerous manoeuvres including a simulated landing approach.
"We all know Paul would have been proud to witness today's historic achievement. The aircraft is a remarkable engineering achievement and we congratulate everyone involved," said the late billionaire's sister Jody Allen, who is also the chair of his philanthropic trust.
