STEM, the integration of science, technology, engineering, and mathematics subjects is a popular topic as schools grapple with how to best prepare students for an ever-evolving society. As societal and technological challenges emerge, design thinking has been lauded as a method to enable people to help tackle those challenges. The steps of the design thinking process, "empathize," "define," "ideate," "prototype" and "test" align with engineering design and can be used as a problem-solving method in classrooms to help promote creativity, critical thinking, and collaboration. The purpose of this explanatory sequential mixed methods study was to better understand if a STEM integrated curriculum helps promote design thinking in middle schoolers. The study compared two middle school groups, one that uses an integrated STEM curriculum and one that does not. Quantitative data was collected using the design thinking disposition survey through pre and post testing. Qualitative data was collected through free response questions and student and teacher interviews. There was no difference found in the change of design thinking dispositions between students at the two schools, however both groups scored lowest on the design thinking disposition of "prototype." Free response questions showed that students at the STEM integrated school perceived an increased ability to design solutions to problems. Student and teacher interviews highlighted benefits of using a STEM integrated curriculum including providing collaborative opportunities to solve hands-on, open-ended problems. How a STEM integrated curriculum can develop design thinking should continue to be examined.