  The M31 X-ray source RX J0042.3+4115 was originally identified as a black
hole binary because it displayed characteristic low state variability at
conspicuously high luminosities; unfortunately, this variability was later
found to be artificial. However, analysis of 84 Chandra ACIS observations, an
HST ACS/WFC observation, and a 60 ks XMM-Newton observation has supplied new
evidence that RX J0042.3+4115 is indeed a black hole binary. The brightest
optical star within 3 sigma of the position of RX J0042.3+4115 had a F435W (~B)
magnitude of 25.4+/-0.2; M_B > -0.4, hence we find a low mass donor likely. RX
J0042.3+4115 was persistently bright over ~12 years. Spectral fits revealed
characteristic black hole binary states: a low/hard state at 2.08+/-0.08 E+38
erg/s, and a steep power law state at 2.41+/-0.05 E+38 erg/s (0.3--10 keV). The
high luminosity low state suggests a ~20 M_Sun primary; this is high, but
within the range of known stellar black hole masses. The inner disk temperature
during the steep power law state is 2.24$\pm$0.15 keV, high but strikingly
similar to that of GRS 1915+105, the only known Galactic black hole binary with
a low mass donor t o be persistently bright. Therefore RX J0042.3+4115 may be
an analog for GRS 1915+105; however, other mechanisms may account for its
behavior. We find compelling evidence for an extended corona during the steep
power law state, because compact corona models where the seed photons for
Comptonization are tied to the inner disc temperature are rejected.
