The recent detection of gravitational waves has proven the existence of massive stellar black hole binaries (BHBs), but the formation channels of BHBs are still an open question. Here, we investigate the demography of BHBs by using our new population-synthesis code MOBSE. MOBSE is an updated version of the widely used binary population-synthesis code BSE (Hurley et al. 2000, 2002) and includes the key ingredients to determine the fate of massive stars: up-to-date stellar wind prescriptions and supernova models. With MOBSE, we form BHBs with total mass up to about 120 Msun at low metallicity, but only systems with total mass up to about 80 Msun merge in less than a Hubble time. Our results show that only massive metal-poor stars (Z < 0.002) can be the progenitors of gravitational wave events like GW150914. Moreover, we predict that merging BHBs form much more efficiently from metal-poor than from metal-rich stars.