This paper reports on an iterative design-based research project to develop an online design tool (the Learning Designer) to support "teachers as designers." The aim is to evaluate the potential of the tool to develop and support a knowledge-building teaching professional community. The Learning Designer was embedded and evaluated through international online "design challenge" events, and a series of MOOCs, providing both quantitative and qualitative data. Findings indicate that the Learning Designer enables an online community of teachers from across the K-12, further and higher education sectors (~400 per day) to build and share their developing knowledge of learning design, and that this would be strengthened by further functionality to support collaboration and peer review of the learning designs created. The research shows how digital technology could bring about large-scale improvements in teacher professional development of TEL. The paper concludes with users' priorities for new features to mobilise community knowledge via large-scale professional development of teachers as innovative TEL designers.