  We evaluate the planar two-loop QCD diagrams contributing to the leading
color coefficient of the heavy-quark pair production cross section, in the
quark-antiquark annihilation channel. We obtain the leading color coefficient
in an analytic form, in terms of one- and two-dimensional harmonic
polylogarithms of maximal weight 4. The result is valid for arbitrary values of
the Mandelstam invariants s and t, and of the heavy-quark mass m. Our findings
agree with previous analytic results in the small-mass limit and numerical
results for the exact amplitude.
