Double-walled carbon nanotubes (DWNTs) can adopt four distinct permutations arising from the electronic type (metallic or semiconducting) of their inner and outer walls. This polydispersity limits the utility of DWNTs in applications such as thin film electronics. We demonstrate that density gradient ultracentrifugation can be employed to address this source of heterogeneity by producing DWNTs with well-defined outer-wall electronic types. Optical absorption measurements of sorted DWNTs reveal outer-wall purities of 96% and 98% for sorted semiconducting and metallic samples, respectively. Electrical characterization of semiconducting and metallic outer-wall DWNTs in thin film transistors directly confirms the efficacy of these separations, with semiconducting DWNT devices yielding on/off ratios 2 orders of magnitude higher than comparable metallic DWNT devices.