A giant six-engine aircraft with the world's longest wingspan completed what company officials called a successful initial flight over California's Mojave Desert, bringing to life a dream held by late Microsoft co-founder Paul Allen.

Key points: The plane has a 117-metre wingspan, longer than any other aircraft in the world

The plane has a 117-metre wingspan, longer than any other aircraft in the world The plane is designed to launch satellites from the air, avoiding the limitations of fixed launch sites

The plane is designed to launch satellites from the air, avoiding the limitations of fixed launch sites The aircraft was built by a company owned by late Microsoft co-founder Paul Allen

Jean Floyd, Stratolaunch Systems chief executive, said the aircraft made a "spectacular" landing that was on the mark.

Stratolaunch, which was founded by Mr Allen, is vying to be a contender in the market for air-launching small satellites. He died in October.

"It was an emotional moment for me, to personally watch this majestic bird take flight, to see Paul Allen's dream come to life in front of my very eyes," Mr Floyd told a teleconference briefing.

Mr Floyd said that as the plane took off, "I did whisper a 'thank you' to Paul for allowing me to be part of this remarkable achievement".

The Stratolaunch jet is designed to carry satellites between its two fuselages. ( Reuters: Gene Blevins )

The behemoth, twin-fuselage Stratolaunch jet took off from Mojave Air and Space Port and climbed into the desert sky 112 kilometres north of Los Angeles.

The jet flew for two and a half hours, achieving a maximum speed of 304 kilometres per hour and altitudes up to 17,000 feet (5,181 meters), the company said.

Test pilot Evan Thomas said the flight overall was "fantastic" and for the most part the plane flew as predicted.

The plan flew for more than two hours, reaching an altitude of 17,000 feet. ( Reuters: Gene Blevins )

"The airplane very nicely, smoothly rotated and really just lifted off the ground," he said.

"It definitely was ready to fly and wanted to fly and climbed out quickly."

Mr Thomas said there were "a few little things that were off-nominal but really for a first flight it was spot on."

The aircraft is a dream of Microsoft co-founder Paul Allen. ( Reuters: Gene Blevins )

The aircraft is designed to carry as many as three satellite-laden rockets at a time under the centre of its enormous wing, which stretches 117 metres — a longer wingspan than any other aircraft.

At an altitude of 35,000 feet (10,668 metres), the rockets would be released, ignite their engines and soar into space.

The advantages of such air-launch systems include being able to use numerous airports and avoid the limitations of fixed launch sites which can be impacted by weather, air traffic and ship traffic on ocean ranges.

While Stratolaunch called its 73-metre-long aircraft the world's largest, other airplanes exceed it in length from nose to tail.

They include the six-engine Antonov AN 225 cargo plane, which is 84 metres long, and the Boeing 747-8, which is just over 76 metres long.

AP