  We consider, at order $\alpha_s^2$ in the QCD coupling, top-quark pair
production in the continuum at various center-of-mass energies and $b$-quark
pair production at the $Z$ resonance by (un)polarized electron and positron
beams. For top quarks we compute the forward-backward asymmetry with respect to
the top-quark direction of flight, the associated polar angle distribution, and
we analyze the effect of beam polarization on the QCD corrections to the
leading-order asymmetry. We calculate also the polarized forward-backward
asymmetry. For $b$-quark production at the $Z$ peak we explore different
definitions of $A_{\rm FB}$. In particular, we analyze $b$ jets defined by the
Durham and the flavor-$k_T$ clustering algorithms. We compute the inclusive
$b$-jet and two-jet asymmetry with respect to the $b$-jet direction. For the
latter asymmetry the QCD corrections to order $\alpha_s^2$ are small. That
predestines it to act as a precision observable.
