  We derive the Standard model gauge group together with chiral fermion
generations from the heterotic string by turning on a Wilson line on a
non-simply connected Calabi-Yau threefold with an SU(5) gauge group. For this
we construct stable ${\bf Z_2}$-invariant $SU(4)\times U(1)$ bundles on an
elliptically fibered cover Calabi-Yau threefold of special fibration type (the
$B$-fibration). The construction makes use of a modified spectral cover
approach giving just invariant bundles.
