  We present an updated mass model for M31 that makes use of a Spitzer 3.6
$\mu$m image, a mass-to-light ratio gradient based on the galaxy's B-R colour
profile, and observed rotation curve data from a variety of sources. We examine
cases where the dark matter follows a pure NFW profile and where an initial NFW
halo contracts adiabatically in response to the formation of the galaxy. We
find that both of these scenarios can produce a reasonable fit to the observed
rotation curve data. However, a pure NFW model requires a concentration
c_{vir}=51 that is well outside the range predicted in LCDM cosmology and is
therefore disfavoured. An adiabatically contracted NFW halo favors an initial
concentration c_{vir}=20 and virial mass 8.2x10^{11} M_{\odot}, and this is in
line with the cosmological expectations for a galaxy of the size of M31. The
best-fit mass is consistent with published estimates from Andromeda Stream
kinematics, satellite galaxy radial velocities, and planetary nebulae studies.
Finally, using the known linear correlation between rotation curve shear and
spiral arm pitch angle, we show that the stellar spiral arm pitch angle of M31
(which cannot be deduced from imaging data due to the galaxy's inclination) is
P=24.7\pm4.4 degrees.
