  Superconductors with the A15 structure are prototypical type-II s-wave
superconductors which have generated considerable interest in early
superconducting material history. However, the topological nature of the
electronic structure remains unnoticed so far. Here, using first-principles
band structure calculations based on density functional theory, we show that
the A15 superconductors (Ta$_3$Sb, Ta$_3$Sn, and Ta$_3$Pb) have nontrivial band
topology in the bulk electronic band structures, leading to the formation of
topological surface states near the Fermi energy. Due to the bulk
superconductivity, the proximity effect in the topological surface states would
induce topological superconductivity even without heterostructure of a
topological insulator and an s-wave superconductor. Our results indicate that
the A15 superconductors are promising candidates for the realization of the
topological superconductivity and the Majorana fermion.
