  We present the first results of an XMM-Newton EPIC observation of the
luminous X-ray source population in the supergiant spiral galaxy M101. We have
studied the properties of the fourteen most luminous sources, all of which have
intrinsic X-ray luminosities exceeding the Eddington limit for a 1.4 solar mass
neutron star, with a subset in the ultraluminous X-ray source (ULX) regime.
Eleven sources show evidence of short-term variability, and most vary by a
factor of ~2-4 over a baseline of 11-24 yrs, providing strong evidence that
these sources are accreting X-ray binary (XRB) systems. The sources show a
variety of spectral shapes, with no apparent spectral distinction between those
above and below the ULX threshold. Nine are well-fit with either simple
absorbed disc blackbody/powerlaw models. However for three of the four sources
best-fit with powerlaw models, we cannot exclude the disc blackbody fits and
therefore conclude that, coupled with their high luminosities, eight out of
nine single-component sources are possibly high state XRBs. The nuclear source
has the only unambiguous powerlaw spectrum (photon index~2.3), which may be
evidence for a low-luminosity AGN. The remaining five sources require at least
two-component spectral fits. We have compared the spectral shapes of nine
sources covered by both this observation and an archival 100ks Chandra
observation of M101; the majority show behaviour typical of Galactic XRBs i.e.
softening with increasing luminosity. We find no definitive spectral signatures
to indicate that these sources contain neutron star primaries, and conclude
that they are likely to be stellar-mass black hole XRBs, with black hole masses
of ~2-23 solar masses if accreting at the Eddington limit (abridged).
