  We address the problem of the origin of massive stars, namely the origin,
path and timescale of the mass flows that create them. Based on extensive
numerical simulations, we propose a scenario where massive stars are assembled
by large-scale, converging, inertial flows that naturally occur in supersonic
turbulence. We refer to this scenario of massive-star formation as the
"Inertial-Inflow Model". This model stems directly from the idea that the mass
distribution of stars is primarily the result of turbulent fragmentation. Under
this hypothesis, the statistical properties of the turbulence determine the
formation timescale and mass of prestellar cores, posing definite constraints
on the formation mechanism of massive stars. We quantify such constraints by
the analysis of a simulation of supernova-driven turbulence in a 250-pc region
of the interstellar medium, describing the formation of hundreds of massive
stars over a time of approximately 30 Myr. Due to the large size of our
statistical sample, we can say with full confidence that massive stars in
general do not form from the collapse of massive cores, nor from competitive
accretion, as both models are incompatible with the numerical results. We also
compute synthetic continuum observables in Herschel and ALMA bands. We find
that, depending on the distance of the observed regions, estimates of core mass
based on commonly-used methods may exceed the actual core masses by up to two
orders of magnitude, and that there is essentially no correlation between
estimated and real core masses.
