  We evaluate the two-photon box contribution to heavy fermion production in
electron positron annihilation, that provides O(alpha^2) electromagnetic
corrections to the Born cross section. The study of its non-relativistic
expansion, relevant at energies close to the threshold of production, is also
performed. We also verify that the threshold expansion of the one-loop
integrals correctly reproduces our results, thus extending the applicability of
this technique to heavy fermion production diagrams.
