We report a search for heavy Majorana neutrinos (N) decaying into a W boson and a lepton using the CMS detector at the Large Hadron Collider. We search for a signature of two same-sign electrons and two jets or two same-sign muons and two jets or a same-sign electron-muon pair and two jets using 19.7 $fb^{-1}$ of data collected during 2012 in proton-proton collisions at a center-of-mass energy of 8 TeV. We find the data to be consistent with the expected standard model background and upper limits are set on ${|V_{lN}|}^{2}$, as a function of heavy Majorana neutrino mass from 40-500 GeV/$c^2$, where $V_{lN}$ is the mixing element of the heavy Majorana neutrino with the standard model neutrino. The upper limits obtained for (${|V_{eN}|}^{2}$, ${|V_{\mu N}|}^{2}$, $|V_{eN}V_{\mu N}^*|$) are (0.022, 0.00470, 0.009) for $m_N$ = 50 GeV/$c^2$, (0.016, 0.0123, 0.009) for $m_N$ = 200 GeV/$c^2$, and (0.72, 0.583, 0.47) for $m_N$ = 500 GeV/$c^2$. These results extend considerably the regions excluded by previous direct searches.