  We present the first "combined stellar structure and atmosphere models"
(CoStar) for massive stars, which consistently treat the entire mass loosing
star from the center out to the asymptotic wind velocity. The models use
up-to-date input physics and state-of-the-art techniques to model both the
stellar interior and the spherically expanding non--LTE atmosphere including
line blanketing. Our models thus yield consistent predictions regarding not
only the basic stellar parameters, including abundances, but also theoretical
spectra along evolutionary tracks. On the same ground they allow us to study
the influence of stellar winds on evolutionary models.
  In this first paper, we present our method and investigate the wind
properties and the interior evolution on the main sequence (MS) at solar
metallicity.
  The wind momentum and energy deposition associated with the MS evolution is
given and the adopted wind properties are discussed. From our atmosphere
calculations, we also derive theoretical estimates of mass loss driven by
radiation pressure. These values are compared with the predictions from recent
wind models of the Munich group. We find an overall agreement with most of
their results. In addition, our models are better in reproducing the strong
wind momentum rates observed in supergiants than those of Puls et al. (1995).
  A comparison between boundary conditions given by the conventional plane
parallel and the new spherically expanding atmosphere approach is made. For the
MS evolution the evolutionary tracks and the interior evolution are found to be
basically unchanged by the new treatment of the outer layers. Given the small
spherical extension of the continuum forming layers in the considered
