The world's largest plane flew for the first time Saturday morning. Developed by a company founded by the late Microsoft co-founder Paul Allen, the Stratolaunch aircraft is an enormous six-engine jet with the wingspan of a football field. It was designed as a kind of flying launching pad from which a rocket loaded with a satellite can launch into low Earth orbit at a cost lower than traditional terrestrial launch. "It was an emotional moment to watch this bird take flight," said Stratolaunch Systems CEO Jean Floyd.