Drones are becoming ubiquitous and offer support to people in various tasks, such as photography, in increasingly interactive social contexts. We introduce drone.io, a projected body-centric graphical user interface for human-drone interaction. Using two simple gestures, users can interact with a drone in a natural manner. drone.io is the first human-drone graphical user interface embedded on a drone to provide both input and output capabilities. This paper describes the design process of drone.io. We present a proof of concept, drone-based implementation, as well as a fully functional prototype for a drone tour-guide scenario. We report drone.io's evaluation in three user studies (N=27) and show that people were able to use the interface with little prior training. We contribute to the field of human-robot interaction and the growing field of human-drone interaction.