The purpose of this paper is to investigate the effects of a curriculum design-based (CDB) professional development model on K-12 teachers' engineering knowledge, attitudes, and behaviors. This teacher professional development approach differs from other training programs where teachers learn how to use a standard curriculum and adopt it in their classrooms. In a CDB professional development model teachers actively design lessons, student resources, and assessments for their classroom instruction. In other science, technology, engineering and mathematics (STEM) disciplines, CDB professional development has been reported to (a) position teachers as architects of change, (b) provide a professional learning vehicle for educators to reflect on practices and develop content knowledge, (c) inspire a sense of ownership in curriculum decision-making among teachers, and (d) use an instructional approach that is coherent with teachers' interests and professional goals. This preliminary study evaluates the effects of a CDB professional development program, the PTC STEM Certificate Program, on twenty-six K-12 teachers who participated in 62 hours of engineering education professional development over a six month period. Participants learned about industry and education engineering concepts, tested engineering curricula, collaborated with K-12 educators and industry professionals, and developed project-based engineering curricula. Data was collected pre-, mid-, and post-program using teacher surveys and a curriculum evaluation instrument. Study results indicate improvements in self-reported engineering subject-matter knowledge and teachers' engineering curriculum design self-efficacy. Analysis of teachers' curricula indicates alignment with multiple educational standards and integration of engineering design strategies.