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Bismuth antimony alloys (Bi_xSb_1-x) provide a tunable materials platform to study topological transport and spin-polarized surface states resulting from the nontrivial bulk electronic structure. In bulk form the small electronic band gap limits the use of Bi_xSb_1-x for applications, however, quantum confinement provides a route to increase the bulk gap. In this work we grow epitaxial, single orientation thin films of Bi_xSb_1-x on InSb(111)B substrate down to a single bilayer. Using spin- and angle-resolved photoemission spectroscopy we show that by reducing film thickness we increase the bulk gap by several hundred meV, while maintaining a topologically nontrivial bulk. We observe a large spin polarization in the topological surface states in ultrathin films. This result is substantiated by a tight binding model that accurately incorporates the matrix element effect. Gated Hall bars are measured at low temperature and high field. The growth and characterization of ultrathin Bi_xSb_1-x alloys reopens the door for this material system to be used in applications such as topological devices, low power electronics, and spintronics.