  It is well known that massive O-stars are frequently (if not always) found in
binary or higher-order multiple systems, but this fact has been less robustly
investigated for the lower mass range of the massive stars, represented by the
B-type stars. Obtaining the binary fraction and orbital parameter distributions
of B-type stars is crucial to understand the impact of multiplicity on the
archetypal progenitor of core-collapse supernovae as well as to properly
investigate formation channels for gravitational wave progenitors. This work
aims to characterise the multiplicity of the B-star population of the young
open cluster NGC 6231 through multi-epoch optical spectroscopy of 80 B-type
stars. We analyse 31 FLAMES/GIRAFFE observations of 80 B-type stars, monitoring
their radial velocities (RVs) and performing a least-squares spectral analysis
(Lomb-Scargle) to search for periodicity in those stars with statistically
significant variability in their RVs. We constrain an observed spectroscopic
binary fraction of $33\pm5$% for the B-type stars of NGC 6231, with a first
order bias-correction giving a true spectroscopic binary fraction of $52\pm8$%.
Out of 27 B-type binary candidates, we obtained orbital solutions for 20
systems: 15 single-lined (SB1) and 5 double-lined spectroscopic binaries
(SB2s). We present these orbital solutions and the orbital parameter
distributions associated with them. Our results indicate that Galactic B-type
stars are less frequently found in binary systems than their more massive
O-type counterparts, but their orbital properties generally resemble those of
B- and O-type stars in both the Galaxy and Large Magellanic Cloud.
