  We present the properties of Milky Way- and Andromeda-like (MW/M31-like)
galaxies simulated within TNG50, the highest-resolution run of the IllustrisTNG
suite of $\Lambda$CDM magneto-hydrodynamical simulations. We introduce our
fiducial selection for MW/M31 analogs, which we propose for direct usage as
well as for reference in future analyses. TNG50 contains 198 MW/M31 analogs,
i.e. galaxies with stellar disky morphology, with a stellar mass in the range
of $M_* = 10^{10.5 - 11.2}$ Msun, and within a MW-like Mpc-scale environment at
z=0. These are resolved with baryonic (dark matter) mass resolution of
$8.5\times10^4$ Msun ($4.5\times10^5$ Msun) and $\sim150$ pc of average spatial
resolution in the star-forming regions: we therefore expand by many factors (2
orders of magnitude) the sample size of cosmologically-simulated analogs with
similar ($\times 10$ better) numerical resolution. The majority of TNG50 MW/M31
analogs at $z=0$ exhibit a bar, 60 per cent are star-forming, the sample
includes 3 Local Group (LG)-like systems, and a number of galaxies host one or
more satellites as massive as e.g. the Magellanic Clouds. Even within such a
relatively narrow selection, TNG50 reveals a great diversity in galaxy and halo
properties, as well as in past histories. Within the TNG50 sample, it is
possible to identify several simulated galaxies whose integral and structural
properties are consistent, one or more at a time, with those measured for the
Galaxy and Andromeda. With this paper, we document and release a series of
broadly applicable data products that build upon the IllustrisTNG public
release and aim to facilitate easy access and analysis by public users. These
include datacubes across snapshots ($0 \le z \le 7$) for each TNG50 MW/M31-like
galaxy, and a series of value-added catalogs that will be continually expanded
to provide a convenient and up to date community resource.
