A set of evolutionary sequences for the two components of massive close binaries in the mass range 9 to 40 M⊙ for mass ratios 0.9 and 0.6 has been calculated. The used code accounts for mass transfer according to case B, i.e. mass exchange after core hydrogen exhaustion, during semi-detached and contact phases. All models were computed for a single initial chemical composition X = 0.7, Z = 0.02. Stellar wind mass losses were taken into account, as well as convective core overshooting (α = 0.25). The Roche lobe overflow was calculated according to a simplified hydrodynamical treatment. For the mixing of infalling material with the originally present layers the process of thermohaline mixing was adopted. It was assumed that 50% of the matter lost by the primary is accreted by the secondary.