  We develop a semi-analytic model to investigate how accretion onto wide
low-mass binary stars can result in a close high-mass binary system. The key
ingredient is to allow mass accretion while limiting the gain in angular
momentum. We envision this process as being regulated by an external magnetic
field during infall. Molecular clouds are made to collapse spherically with
material either accreting onto the stars or settling in a disk. Our aim is to
determine what initial conditions are needed for the resulting binary to be
both massive and close. Whether material accretes, and what happens to the
binary separation as a result, depends on the relative size of its specific
angular momentum, compared to the specific angular momentum of the binary. When
we add a magnetic field we are introducing a torque to the system which is
capable of stripping the molecular cloud of some of its angular momentum, and
consequently easing the formation of high-mass binaries. Our results suggest
that clouds in excess of 1000 M$_\odot$ and radii of 0.5 pc or larger, can
easily form binary systems with masses in excess of 25 M$_\odot$ and
separations of order 10 R$_\odot$ with magnetic fields of order 100 {\mu}G
(mass-to-flux ratios of order 5).
