  Black hole (BH) X-ray binaries (XRBs) are X-ray luminous binary systems
comprising a BH accreting matter from a companion star. Understanding their
origins sheds light on the still not well understood physics of BH formation.
M33 X-7 hosts one of the most massive stellar-mass BH among all XRBs known to
date, a 15.65 Msun BH orbiting a 70 Msun companion star in a 3.45 day orbit.
The high masses of the two components and the tight orbit relative to the large
H-rich stellar component challenge our understanding of the typically invoked
BH-XRBs formation channels. The measured underluminosity of the optical
component further complicates the picture. A solution to the evolutionary
history of this system that can account for all its observed properties has yet
to be presented, and here we propose the first scenario that is consistent with
the complete set of current observational constraints. In our model, M33 X-7
started its life hosting a 85-99 Msun primary and a 28-32 Msun companion in a
Keplerian orbit of 2.8-3.1 days. In order to form a BH of 15.65 Msun, the
initially most massive component transferred part of its envelope to the
companion star and lost the rest in a strong stellar wind. During this
dynamically stable mass transfer phase the companion accreted matter, to become
the presently underluminous 70 Msun star.
