A search is presented for a singly produced excited bottom quark (b$^{}$) decaying to a top quark and a W boson in the all-hadronic, lepton+jets, and dilepton final states in proton-proton collisions at $ \sqrt{s}=8 $ TeV recorded by the CMS experiment at the CERN LHC. Data corresponding to an integrated luminosity of 19.7 fb$^{−1}$ are used. No significant excess of events is observed with respect to standard model expectations. We set limits at 95% confidence on the product of the b$^{}$ quark production cross section and its branching fraction to tW. The cross section limits are interpreted for scenarios including left-handed, right-handed, and vector-like couplings of the b$^{}$ quark and are presented in the two-dimensional coupling plane based on the production and decay coupling constants. The masses of the left-handed, right-handed, and vector-like b$^{}$ quark states are excluded at 95% confidence below 1390, 1430, and 1530 GeV, respectively, for benchmark couplings. This analysis gives the most stringent limits on the mass of the b$^{∗}$ quark to date.