In 2009, the National Science Foundation funded the "Dynabook: A Digital Resource and Preservice Model for Developing TPCK" project through its Discovery Research K-12 program. Dynabook project leaders and the National Science Foundation (NSF) recognized that digital textbooks would soon be a primary instructional resource, and seized the opportunity to pursue the development of an innovation deliberately and thoughtfully--not to simply recreate a textbook in a digital format. Prior to Dynabook, there were no well-designed, well-organized digital instructional resources for preservice programs that were also aligned with the Common Core standards in mathematics. The original purpose of the Dynabook project was to design a dynamic and adaptable resource for middle school mathematics teaching that would incorporate key elements of two frameworks--the Universal Design for Learning (UDL) framework and the Technological Pedagogical Content Knowledge (TPCK) framework. To achieve this goal, the Dynabook project assembled a multidisciplinary team with expertise in these two frameworks, as well as in technology development: SRI International in Menlo Park, CA; CAST (the Center for Applied Special Technology) in Wakefield, MA; and San Francisco and San Diego State Universities in California as organizational partners; and mathematicians, computer scientists, teacher educators, and learning scientists as individual partners. At its core, Dynabook is now a digital resource to help teacher educators engage their students (who are middle school mathematics teacher candidates) in cognitively demanding proportional reasoning. It helps teachers to use an appropriate pedagogy when teaching proportional reasoning, and to collect and consider evidence of their students' understanding of proportionality. This paper focuses on the engineering design of the project itself and describes the research and development process. "About the Teacher Educator Network" is appended.