In this paper, we introduce the User Interface Pilot, a model-based software tool that enables designers and engineers to create the initial specifications for the pages of a website, or for the screens of a desktop or mobile application. The tool guides the design of these specifications, commonly known as wireframes, in a user-centered fashion by framing the context of the design within the concepts of user tasks, user types, and data objects. Unlike previous model-based tools, the User Interface Pilot does not impose a rigid model-driven methodology and functions well within common software engineering development processes. The tool has been used in over twenty real-world user interface design projects.