  We provide an extensive set of theoretical spectral energy distributions of
massive stars derived from our "combined stellar structure and atmosphere
models". The calculations covering the entire main sequence evolution for
initial masses M_i=20 - 120 Msun (O3-B0 stars of all luminosity classes) are
used for a systematic study of the ionizing fluxes of O and early B stars. We
demonstrate the importance of accounting simultaneously for non-LTE effects,
line blanketing and stellar winds. The main results from our spectra are the
following: (1) The flux in the HeII continuum is increased by 2 to 3 (3 to 6)
orders of magnitudes compared to predictions from plane parallel non-LTE (LTE)
model atmospheres. (2) The flux in the HeI continuum is increased due to
non-LTE effects. However, we find that it is also influenced by wind effects.
The combined effect of a mass outflow and line blanketing leads to a flatter
energy distribution in the HeI continuum, which confirms the results of
Sellmaier et al. (1996) for a wider range of stellar parameters. (3) The flux
in the Lyman continuum is also modified due to line blanketing and the presence
of a stellar winds, although to a lesser degree than the spectrum at higher
energies. We derive revised ionizing fluxes for O3 to B0 stars based on the
recent Teff and gravity calibrations of Vacca et al. (1996). The total number
of Lyman continuum photons is found to be slightly lower than previous
derivations. Due to the increased flux in the HeI continuum the hardness ratio
of the HeI to H continuum is increased by 1.6 to 2.5 depending on spectral type
and luminosity class. A critical discussion of current model assumptions shows
that for stars of types later than B0, having weak stellar winds, reliable
predictions of ionizing fluxes are not yet possible.
