X stands for the unknown in mathematics and physics. An `X-Plane' is an experimental or research aircraft designed to explore the boundaries of aeronautical science, and the term has become shorthand for any developmental model, research aircraft or technology demonstrator. This book details 150 of these incredible and unusual aircraft, from the early years of aviation to the present day, exploring their capabilities, deatures and development. Each aircraft is illustrated with a full page photograph, annotated to show particularly unusual or experimental features, while the lively text provides an insight into the history and test career of each type, alongside archival images from the aircraft's heyday, artwork or diagrams....