A first measurement of the top-quark mass is presented based on the peak position of the energy spectrum of b jets produced from top-quark decays. This novel technique follows a recent theoretical proposal aiming to minimize systematic uncertainties related to the modeling of top quark production. The analysis is performed selecting top-antitop events with electron-muon final states in proton-proton collision data at $\sqrt{s}$ = 8 TeV with the CMS detector, corresponding to an integrated luminosity of 19.7 fb$^{-1}$. The energy peak position is obtained by fitting the observed energy spectrum, and is translated to a top-quark mass estimation using relativistic kinematics, calibrated with Monte Carlo simulation.