From a distance, it looks like a war plane imagined by Hayao Miyazaki, or the type of machine that could only exist in a science fiction novel. But according to aviation enthusiasts who captured footage of it soaring in the California skies last week, it’s an experimental aircraft designed by Stratolaunch, an aerospace company located in the Mojave Desert. Chances are, residents in surrounding towns will see more of it, too. Described as the world’s largest flying aircraft, Roc — which looks like two 747s fused together and has a wingspan larger than a football field — recently completed a...