A search for long-lived heavy neutrinos (N) in the decays of B mesons produced in proton-proton collisions at$$ \sqrt{s} $$$\sqrt{s}$= 13 TeV is presented. The data sample corresponds to an integrated luminosity of 41.6 fb⁻¹collected in 2018 by the CMS experiment at the CERN LHC, using a dedicated data stream that enhances the number of recorded events containing B mesons. The search probes heavy neutrinos with masses in the range 1 <m_N< 3 GeV and decay lengths in the range 10⁻²<cτ_N< 10⁴mm, where τ_Nis the N proper mean lifetime. Signal events are defined by the signature B →ℓ_BNX; N →ℓ^±π^∓, where the leptonsℓ_Bandℓcan be either a muon or an electron, provided that at least one of them is a muon. The hadronic recoil system, X, is treated inclusively and is not reconstructed. No significant excess of events over the standard model background is observed in any of theℓ^±π^∓invariant mass distributions. Limits at 95% confidence level on the sum of the squares of the mixing amplitudes between heavy and light neutrinos, |V_N|², and oncτ_Nare obtained in different mixing scenarios for both Majorana and Dirac-like N particles. The most stringent upper limit|V_N|²< 2.0×10⁻⁵is obtained atm_N= 1.95 GeV for the Majorana case where N mixes exclusively with muon neutrinos. The limits on|V_N|²for masses 1 <m_N< 1.7 GeV are the most stringent from a collider experiment to date.