Integrating science, technology, engineering, art, and mathematics (STEAM) education has become an emphasis in many countries' K-12 curricula. Design-based learning is an instructional approach that allows students to learn STEAM in more integrated ways than the traditional method. Using design thinking as a pedagogical framework, design-based learning engages students in designing solutions to real-life and human-centred problems. In this process, students must use creativity, visual arts, and design thinking as well as disciplinary knowledge and skills. Of great importance here is creative confidence -- a belief in one's own creative ability. However, research investigating the influence of design-based learning on students' creative confidence is still scare. This mixed-methods study examined 31 eighth-grade students' creative confidence before and after being challenged to design user-friendly tables. This design-based learning lasted for 12 50-minute periods that were scheduled over four weeks. We used a Likert-type questionnaire to collect quantitative data and recorded classroom discourse to gather qualitative data. Using non-parametric statistics, we found that the students significantly improved their creative confidence. Using a content analysis method, we identified crucial sources of this improvement and thus formed recommendations for how to foster students' creative confidence within the context of design-based learning.