Purpose: Engineering students gain confidence and competency through continual practice of key skills. The social cognitive theory construct of self-efficacy provides a useful measure to assess students' beliefs in their ability to succeed or perform tasks. Research focused on the impacts of curricular engineering design experiences on student self-efficacy would provide a basis for intentional and effective improvements to engineering curricula. Accordingly, our work addresses the central research question: To what extent and in what ways does continuous exposure to hands-on biomedical design projects throughout a curriculum influence self-efficacy of biomedical engineering students? Methods: We used a mixed methods approach to investigate two specific research questions: (RQ1) Do biomedical engineering students exhibit changes in self-efficacy when continuously challenged with biomedical design situations? (RQ2) How do biomedical engineering students describe their sources of self-efficacy toward biomedical design throughout a curriculum? Student responses (n = 503) to a quantitative self-efficacy survey were collected in second-, third-, and fourth-year undergraduate biomedical engineering courses with embedded design projects. We performed confirmatory factor analyses to verify model fit and develop weighted mean sum score equations for four constructs: general engineering self-efficacy (GSE), experimental self-efficacy (ESE), tinkering self-efficacy (TSE), and design self-efficacy (DSE). We employed the structural equation model to study changes in student self-efficacy both across the curriculum and within individual courses. We also developed a qualitative survey mapped to Bandura's four identified sources of self-efficacy (mastery experiences, vicarious experiences, social persuasion, and emotional state) and performed thematic analysis on student responses (n = 216) to identify sources of student self-efficacy. Results: Biomedical engineering students exhibit significant gains in GSE, ESE, TSE, and DSE across the three years of our undergraduate program (i.e., after general first-year engineering pre-requisites), with the largest gains observed in DSE. Paired pre/post data from specific courses demonstrate student gains in DSE in response to integrated design projects. In qualitative survey responses, most students identified mastery experiences, especially prior coursework and design projects (68%), that prepared them for design. Comparably fewer students described vicarious experiences that contributed to their design readiness, as some mentioned learning skills (37%) or knowledge (19%) from their peers. Most students (65%) cited external factors that affected their emotional states and design contributions, while fewer mentioned social persuasions, such as encouragements from their instructors and teaching assistants (17%) or peers (16%). Conclusion: Biomedical engineering students exhibit significant gains in self-efficacy across a design-rich undergraduate curriculum. Their identified sources of self-efficacy provide guidance for engineering educators who aim to improve design instruction and curricula.