  We present synthetic spectra and SEDs computed along evolutionary tracks at
Z=1/5 Zsun and Z=1/30 Zsun, for masses between 15 and 150 Msun. We predict that
the most massive stars all start their evolution as O2 dwarfs at sub-solar
metallicities. The fraction of lifetime spent in the O2V phase increases at
lower metallicity. The distribution of dwarfs and giants we predict in the SMC
accurately reproduces the observations. Supergiants appear at slightly higher
effective temperatures than we predict. More massive stars enter the giant and
supergiant phases closer to the ZAMS, but not as close as for solar
metallicity. This is due to the reduced stellar winds at lower metallicity. Our
models with masses higher than ~60 Msun should appear as O and B stars, whereas
these objects are not observed, confirming a trend reported in the recent
literature. At Z=1/30 Zsun, dwarfs cover a wider fraction of the MS and giants
and supergiants appear at lower effective temperatures than at Z=1/5 Zsun. The
UV spectra of these low-metallicity stars have only weak P-Cygni profiles. HeII
1640 sometimes shows a net emission in the most massive models, with an
equivalent width reaching ~1.2 A. For both sets of metallicities, we provide
synthetic spectroscopy in the wavelength range 4500-8000 A. This range will be
covered by the instruments HARMONI and MOSAICS on the ELT and will be relevant
to identify hot massive stars in Local Group galaxies with low extinction. We
suggest the use of the ratio of HeI 7065 to HeII 5412 as a diagnostic for
spectral type. We show that this ratio does not depend on metallicity. Finally,
we discuss the ionizing fluxes of our models. The relation between the hydrogen
ionizing flux per unit area versus effective temperature depends only weakly on
metallicity. The ratios of HeI and HeII to H ionizing fluxes both depend on
metallicity, although in a slightly different way.
