While writing courses often include instruction in rhetorical aspects of writing (i.e., learning to write), business content courses often assign writing as a tool for learning and assessing content knowledge (i.e., writing to learn), with little attention to students' rhetorical understanding of genres. This leaves students with an incomplete understanding of disciplinary genres and at a disadvantage in the workplace. We argue for a "writing-as-design" conceptualization of writing, which crosses the boundary between learning to write and writing to learn by helping students develop an awareness of purpose and reader experience within a disciplinary content course. Using a case study of business plan writing in a business content course, we examine instructional materials, interviews, and student writing to illustrate how business courses can leverage a writing-as-design conceptualization to improve student writing. [For the complete volume, "Crossing Boundaries in Researching, Understanding, and Improving Language Education: Essays in Honor of G. Richard Tucker. Educational Linguistics. Volume 58," see ED672546.]