  We study the B-parameter (``bag factor'') for B-anti-B mixing within a
recently developed heavy-light chiral quark model. Non-factorizable
contributions in terms of gluon condensates and chiral corrections are
calculated. In addition, we also consider 1/m_Q corrections within heavy quark
effective field theory. Perturbative QCD effects below \mu = m_b known from
other work are also included. Considering two sets of input parameters, we find
that the renormalization invariant B-parameter is B = 1.51 +- 0.09 for B_d and
B = 1.40 +- 0.16 for B_s.
