The world's biggest plane is a step closer to its first flight.
Named Stratolaunch, the colossal aircraft successfully fired all six of its Pratt and Whitney turbofan engines - each weighing 8,940lbs (4,000kg) - for the first time this week.
The plane is the vision of Microsoft co-founder Paul Allen who wants it to act as a giant air pad in the sky, allowing payloads to reach space faster and at a lower cost than existing technologies.
The aircraft is so huge that if the it sat in the centre of a football field, it would be wide enough for its wings to reach 12.5 feet (3.8 metres) beyond each goalpost.
Instead of a satellite, the Stratolaunch airplane could launch a Dream Chaser spaceship. This could act as a mini-shuttle to reach low Earth orbit destinatations and return astronauts or payloads to. runway within 24 hours.
Test flights were expected for 2016 and 2017, but project delays have pushed back the date to sometime in 2019.
Scroll down for video
The Stratolaunch aircraft is one step closer to achieving its aim of providing convenient, reliable, and routine access to low Earth orbit, after a milestone breakthrough. Engineers at the firm started all six of the plane's Pratt & Whitney turbofan engines for the first time on Tuesday.
The Stratolaunch weighs approximately 500,000 pounds without any cargo.
It is designed to carry a maximum takeoff weight of 1.3 million pounds, according to The Verge.
The plane rolls around with the aid of 28 wheels. Once airborne, it is powered by six 747 aircraft engines.
The size of the plane will enable it to serve as an airborne rocket launcher.
Traditionally, satellites and other aircraft have been launched into space from a launchpad. This requires a tremendous amount of fuel.
The Stratolaunch, on the other hand, will enable rockets to have a 'head start' since they will be carried into the sky before they launch into space.
The Stratolaunch team completed fuel testing of all six fuel tanks to ensure their proper operations at the company’s facility at the Mojave Air and Space Port in California.
Each of the six tanks were filled independently to check their fuel mechanisms were working correctly and to that they were properly sealed.
In addition to fuel testing, engineers began testing the flight control system.
So far, they have have exercised the full limits of motion and rate of deflection of the wings control surfaces and stabilisers.
Building up to this week’s engine tests, electrical, pneumatic, and fire detection systems were also given a once over.
Writing on the Stratolaunch website, CEO Jean Floyd said: 'Engine testing was conducted with a build-up approach and consisted of three phases.
'First as a "dry motor", where we used an auxiliary power unit to charge the engine.
'Next, as a "wet motor" where we introduced fuel. 'Finally, each engine was started one at a time and allowed to idle.
'In these initial tests, each of the six engines operated as expected.'
Over the next few months, they plan to continue to test the aircraft’s engines at higher power levels and varying configurations, culminating in the start of taxi tests on the runway.
Paul Allen unveiled the world's largest aircraft at the start of June.
The Stratolaunch, seen above being rolled out of its hangar in California's Mojave Desert, is an aircraft that is designed to carry rockets between its two fuselages
Over the next few months, they plan to continue to test the aircraft’s engines at higher power levels and varying configurations, culminating in the start of taxi tests on the runway
The graphic above illustrates the Stratolaunch's wingspan compared to other aircraft models, including the Boeing 747-8 Intercontinental and the Airbus A380-800
The massive plane rolled out by Allen's aerospace firm, Stratolaunch Systems, features the longest wingspan of any aircraft ever built, according to Popular Mechanics.
With a wingspan of 385 feet, the six-engine plane will be larger than Howard Hughes' 1947 H-4 Hercules, known as the 'Spruce Goose,' and the Antonov An-225, a Soviet-era cargo plane originally built to transport the Buran space shuttle that is currently the world's largest aircraft.
Last year, Stratolaunch signed a deal with aerospace and defense firm Orbital ATK.
Under terms of the deal, the Stratolaunch will propel Orbital's Pegasus XL rocket, which is used to send small satellites into space.
Allen's move coincides with a surge of new businesses planning to sell internet access, Earth imagery, climate data and other services from networks of hundreds of satellites in low-altitude orbits around Earth.
But his vision is different from what Elon Musk's SpaceX, Jeff Bezos' Blue Origin, Richard Branson's Virgin Galactic and other companies have for building commercial highways to space.
Musk's goal is to fly people to Mars. Bezos is developing low-cost, reusable rockets with the goal of moving energy-intensive, heavy industry off Earth. Branson is focused on space tourism and a small satellite launcher.
The advantage of Allen's approach will be the ability to position the plane so satellites can be directly delivered into very precise orbits and do so quickly, without launch range scheduling issues and weather-related delays, Chuck Beames, who oversees Allen's space ventures, said.
The Stratolaunch is an aircraft that is designed to carry rockets between its two fuselages.
In 2011, the project's cost was initially estimated to be at $300million, though there is no word as to the updated figures.
After the plane reaches altitude, it would then drop the launch vehicle, which will subsequently fire its boosters and launch into space from the air.