A great video of the US unveiling the Worlds Largest aircraft that will help to boost US Military power. Stratolaunch Systems Corporation is a space transportation venture developing a new air launch to orbit system, with its corporate headquarters located in Seattle, Washington. The project was officially announced in December 2011 by Microsoft co-founder Paul G. Allen and Scaled Composites founder Burt Rutan, who had previously collaborated on the creation of SpaceShipOne.
The project is a mobile launch system with three primary components; a carrier aircraft being built by Scaled Composites, a multi-stage payload "launch vehicle" which would be launched at high altitude into space from under the carrier aircraft, plus a mating and integration system by Dynetics.
The first test flights are planned to begin in 2017, with a goal of a commercial launch by 2020.
The Scaled Composites Model 351 (nicknamed the "Roc") is being built for Stratolaunch Systems to provide a platform from which air-launch space missions can be staged. With a wingspan of 117 m (385 ft), the design has the longest wingspan of any airplane to date (June 2017).
In August 2015, Scaled Composites president Kevin Mickey stated the company has so far assembled "roughly 200,000 pounds [91,000 kg] of composite structure" for the vehicle and if put on an American football field, "its wingtips would extend beyond the goalposts by 15 feet [4.6 m] on each side".
Each of the twin fuselages of the aircraft is 73 m (238 ft) long and is supported by 12 main landing gear wheels and two nose gear wheels, for a total of 28 wheels. It will require 3,700 m (12,000 ft) of runway to lift-off.
The air-launch altitude is planned for about 11,000 m (35,000 ft). Payload is noted as in excess of 230,000 kg (500,000 lb).
As of October 2016 "Orbital ATK will supply multiple Pegasus XL rockets for Stratolaunch to mount underneath the company’s huge carrier aircraft currently under construction in Mojave, California".
The aircraft was rolled out on 31 May 2017.
The Model 351 has the longest wing span of any aircraft yet built, at 385 feet (117 m).  It is of twin-fuselage configuration, similar to the Scaled Composites White Knight Two. The centre section of the high-mounted, high aspect ratio wing is fitted with a Mating and Integration System (MIS) capable of handling a 230,000 kg (500,000 lb) load and being developed by Dynetics. Each fuselage has its own tail with horizontal and vertical stabilizer, leaving a clear area behind the payload to reduce the risk of interference during flight. Three engines are positioned on pylons outboard of each fuselage. The cockpit is positioned within the starboard fuselage.[clarification needed]
The aircraft is powered by six 250 kN (56,000 lbf) Pratt & Whitney PW4056 engines
To cut development costs, many of the aircraft systems have been adopted from the Boeing 747-400, including the engines, avionics, flight deck, landing gear and other systems. Two former United Airlines Boeing 747-400 aircraft (Serial numbers 28715 & 28716) were acquired and taken to the Mojave Air & Space Port for cannibalization.
The completed aircraft, ready for initial ground and fueling tests, was rolled out on May 31, 2017. 
Specifications (Stratolaunch Systems Carrier)
Wingspan comparison of the Stratolaunch carrier with other large airplanes
Data from Stratolaunch
Length: 238 ft (73 m)
Wingspan: 385 ft (117 m)
Gross weight: 1,300,000 lb (589,670 kg)
external payload: 500,000 lb (230,000 kg)
Powerplant: 6 × Pratt & Whitney PW4056 turbofan, 56,750 lbf (252.4 kN) thrust each
Range: 2,000 nmi (2,302 mi; 3,704 km)
The project was started nearly a year prior to the December 2011 public announcement, Dynetics began work in early 2011 and had approximately 40 employees working on the project at the time of the announcement, SpaceX efforts began only shortly prior to the public announcement. The mothership is named by its Scaled model number: M351.
The Stratolaunch Carrier Aircraft was built in a specially-constructed hangar in Mojave, California. The first of two manufacturing buildings, a 88,000 square feet (8,200 m2) facility for construction of the composite sections of the wing and fuselage, was opened for production in October 2012. By June 2016 Scaled Composites had 300 people working on the project.
On May 31, 2017 the first Stratolaunch Carrier Aircraft was towed out of the Stratolaunch Mojave building to start ground testing. The plan is have the first launch in 2019.