We consider the lepton number violating decays B -> mu(+/-) mu(+/-) pi(-/+) and B -> D-(*()) mu(+/-) mu(+/-) pi(-/+) which may be detected at the LHCb and Belle-II experiments and B -> mu(+/-) mu(+/-) e(-/+) nu and B -> D-(*()) mu(+/-) mu(+/-) e(-/+) nu decays which may be detected at the Belle-II experiment. The projected total number of produced B mesons is 4.8 x 10(12) at the LHCb upgrade and 5 x 10(10) at Belle-II. For the case in which the above decays are not detected, we deduce the new upper bounds (sensitivity limits) for the mixing parameter vertical bar U-mu N vertical bar(2) of heavy sterile neutrino with sub-eV light neutrino, as a function of the sterile neutrino mass in the interval 1.75 GeV < M-N < 5.0 GeV. We take into account the probability of decay of the sterile neutrino N within the detector, taking as the effective detector length L = 2.3m at the LCHb upgrade and L = 1m at Belle-II. In the interval 1.75 GeV < M-N < 3 GeV, the most stringent bounds can be obtained with the decays B -> mu(+/-) mu(+/-) pi(-/+) at the LHCb upgrade. The sensitivity limits are expected to be, in general, more stringent at the LHCb upgrade than at Belle-II, principally because the number of produced B mesons in the LHCbupgrade is expected to be about 2 orders of magnitude larger than at Belle-II. We conclude that the LHCb upgrade and Belle-II experiments have the potential to either find a new heavy Majorana neutrino N or to improve significantly the sensitivity limits (upper bounds) on the heavy-light mixing parameter vertical bar U-mu N vertical bar(2), particularly in the mass range 1.75 GeV < M-N < 3 GeV. This work is a continuation and refinement of our previous work [Phys. Rev. D 94, 053001 (2016); Phys. Rev. D 95, 039901(E) (2017)] on the subject.