Searches for heavy Majorana neutrinos in B− decays in final states containing hadrons plus a μ−μ− pair have been performed using 0.41  fb−1 of data collected with the LHCb detector in proton-proton collisions at a center-of-mass energy of 7 TeV. The D+μ−μ− and D*+μ−μ− final states can arise from the presence of virtual Majorana neutrinos of any mass. Other final states containing π+, D+s, or D0π+ can be mediated by an on-shell Majorana neutrino. No signals are found and upper limits are set on Majorana neutrino production as a function of mass, and also on the B− decay branching fractions.