A search for supersymmetry in final states with b jets and large transverse momentum imbalance is presented. The search is carried out in a data sample collected with the CMS detector of proton-proton collisions delivered by the LHC at a centre-of-mass energy of 7~TeV, corresponding to an integrated luminosity of 35~pb$^{-1}$. A total of $0.33^{+0.43}_{-0.33}$~(stat)~$\pm$~0.13~(syst) events is predicted directly from data to arise from standard model processes, and one event is observed in the data. Upper limits are set at the 95\% confidence level on the cross sections of benchmark supersymmetry models.