This study reports on the development of flexible boron nitride nanotube (BNNT) fabrics and significant improvements to their anisotropic thermal conductivity and mechanical toughness. Through BNNT purification and isotopic enrichment with ¹¹B the in-plane thermal conductivity was improved by nearly five times (1.80 to 8.26 W m^-1 K^-1)  while maintaining a low through-plane thermal conductivity (0.13 W m^-1 K^-1). This resulted in an extremely high anisotropic ratio of 62 which is the highest known anisotropy and magnitude in thermal conductivity reported for a bulk BNNT material. Mechanical testing revealed that BNNT purification improved fabric strength and toughness by factors of two and three, respectively due to BNNT bundling and alignment. This study demonstrates the importance that BNNT purity plays in thermal and mechanical properties of bulk BNNT fabrics. As a highly porous nanomaterial, BNNT fabrics have great potential as composite preforms or, given their flexibility and toughness, as standalone high temperature, electrically insulating, thermal management materials.