  We present the largest sample to-date of intermediate-resolution blue-to-red
optical spectra of B-type supergiants in M31 and undertake the first survey of
diffuse interstellar bands (DIBs) in this galaxy. Spectral classifications,
radial velocities and interstellar reddenings are presented for 34 stars in
three regions of M31. Radial velocities and equivalent widths are given for the
5780 and 6283 DIBs towards 11 stars. Equivalent widths are also presented for
the following DIBs detected in three sightlines in M31: 4428, 5705, 5780, 5797,
6203, 6269, 6283, 6379, 6613, 6660, and 6993. All of these M31 DIB carriers
reside in clouds at radial velocities matching those of interstellar Na I
and/or H I. The relationships between DIB equivalent widths and reddening
(E(B-V)) are consistent with those observed in the local ISM of the Milky Way.
Many of the observed sightlines show DIB strengths (per unit reddening) which
lie at the upper end of the range of Galactic values. DIB strengths per unit
reddening are found (with 68% confidence), to correlate with the interstellar
UV radiation field strength. The strongest DIBs are observed where the
interstellar UV flux is lowest. The mean Spitzer 8/24 micron emission ratio in
our three fields is slightly lower than that measured in the Milky Way, but we
identify no correlation between this ratio and the DIB strengths in M31.
Interstellar oxygen abundances derived from the spectra of three M31 H II
regions in one of the fields indicate that the average metallicity of the ISM
in that region is 12 + log[O/H] = 8.54 +- 0.18, which is approximately equal to
the value in the solar neighbourhood.
