  CONTEXT: For more than two decades, stellar atmosphere codes have been used
to derive the stellar and wind parameters of massive stars. Although they have
become a powerful tool and sufficiently reproduce the observed spectral
appearance, they can hardly be used for more than measuring parameters. One
major obstacle is their inconsistency between the calculated radiation field
and the wind stratification due to the usage of prescribed mass-loss rates and
wind-velocity fields. AIMS: We present the concepts for a new generation of
hydrodynamically consistent non-local thermodynamical equilibrium (non-LTE)
stellar atmosphere models that allow for detailed studies of radiation-driven
stellar winds. As a first demonstration, this new kind of model is applied to a
massive O star. METHODS: Based on earlier works, the PoWR code has been
extended with the option to consistently solve the hydrodynamic equation
together with the statistical equations and the radiative transfer in order to
obtain a hydrodynamically consistent atmosphere stratification. In these
models, the whole velocity field is iteratively updated together with an
adjustment of the mass-loss rate. RESULTS: The concepts for obtaining
hydrodynamically consistent models using a comoving-frame radiative transfer
are outlined. To provide a useful benchmark, we present a demonstration model,
which was motivated to describe the well-studied O4 supergiant Zeta Pup. The
obtained stellar and wind parameters are within the current range of literature
values. CONCLUSIONS: For the first time, the PoWR code has been used to obtain
a hydrodynamically consistent model for a massive O star. This has been
achieved by a profound revision of earlier concepts used for Wolf-Rayet stars.
The velocity field is shaped by various elements contributing to the radiative
acceleration, especially in the outer wind. (...)
