This report describes a teaching development grant (TDG) project that adopted a design-based research approach to enhance the language assessment literacy of preservice English teachers. In small groups, 92 preservice teachers engaged in test development projects, during which they developed tailor-made language tests for collaborating schools, implemented the tests, analysed responses, evaluated the tests, provided feedback to the schools, and wrote project reports. The assessment tasks they produced were annotated with detailed task features, categorised according to target skills, students and schools and stored in an online database open to incoming students and the world. 48 participants and users of the database evaluated their experience and the database via a questionnaire, which found a generally positive attitude towards the innovation but also several remaining issues to be addressed in the next iteration of the project. This project and the design-based research approach it adopted are applicable to preservice teacher training in other educational contexts as well as other areas of research interest.