The liquid phase processing including dispersion, purification, and assembly of boron nitride nanotubes (BNNTs) is important for effectively translating unique properties of BNNTs into assembled solid materials, such as films and fibers. Short single-stranded DNA (ssDNA) has been the biopolymer of choice to achieve efficient sorting and assembly of carbon nanotubes, which are structural analogues to BNNTs. Here, we demonstrated a highly efficient aqueous dispersion of BNNTs using (GT)20 ssDNA and the subsequent removal of non-nanotube impurities from dispersions of as-synthesized BNNT material by a membrane filtration approach. The surface coating of BNNTs by DNA creates uniform dispersions at a nanotube concentration of as high as ≈11.5 mass %, leading to the formation of solid aligned films after solvent evaporation without applied shear, therefore demonstrating the first example of BNNT films produced by spontaneous alignment of nanotubes. Our findings provide important insights for producing macroscopic assemblies of aligned BNNT films for potential applications in thermal interface material and electronic and optoelectronic devices.