This paper describes a three-year, design-based research project to redesign a year-long, project-based advanced placement environmental science course to better support student engagement and the development of environmental citizen identities. In the initial implementation, students' increased understanding of environmental problems paradoxically led to disengagement as students felt pessimistic and powerless. We describe design cycles across three implementation years and investigate the impact of design features on engagement and identity. Curricular design features (positioning students as change agents and widening projects' spheres of influence from local to global), alongside expansive framing for transfer, contributed to engagement and the development of practice-linked identities as environmental citizens. We discuss implications for designing courses for engagement and identification with disciplinary content.