The Framework for K-12 Science Education and the Next Generation Science Standards propose that students learn core ideas and practices related to engineering as well as science. To do so, students will need high-quality curricular materials designed to meet these goals. We report an efficacy study of an elementary engineering curriculum, Engineering is Elementary (EiE) that includes a set of hypothesized critical components designed to encourage student engagement in practices, connect engineering and science learning, and reach diverse students. To measure the impact of the curriculum, we conducted a cluster randomized controlled trial in 604 classrooms in 152 schools in three states. Schools were randomly assigned to either the treatment curriculum or to a comparison curriculum that addressed the same learning goals but did not include several critical components. Results show that students who used the treatment curriculum (EiE) regardless of demographic characteristics outperformed students in the comparison group on outcome measures of both engineering and science content learning. The results show that curriculum design affects student-learning outcomes.