  We present a new method to extract statistical constraints on the progenitor
properties and formation channels of individual gravitational-wave sources.
Although many different models have been proposed to explain the binary black
holes detected by the LIGO Scientific and Virgo Collaboration (LVC), formation
through isolated binary evolution remains the best explored channel. Under the
assumption of formation through binary evolution, we use the statistical
wrapper dart_board coupled with the rapid binary evolution code COSMIC to model
the progenitor of GW150914, the first gravitational-wave signal detected by the
LVC. Our Bayesian method combines the likelihood generated from the
gravitational-wave signal with a prior describing the population of stellar
binaries, and the Universe's star-formation and metallicity evolution. We find
that the dominant evolutionary channel for GW150914 did not involve a
common-envelope phase, but instead the system most probably (70%-90%) formed
through stable mass transfer. This result is robust against variations of
various model parameters, and it is reversed only when dynamical instability in
binaries becomes more likely when a strict condition favoring common envelopes
is adopted. Our analysis additionally provides a quantitative description of
the progenitors relevant to each channel.
