[en] We present an analysis of an extensive set of spectroscopic observations of the mysterious early-type binary BD +40 deg4220. A new orbital solution is derived from the radial velocities of the absorption lines. We confirm that the secondary star is highly overluminous for its mass. The absorption lines of both components display phase-locked profile variations, with some of the secondary's lines going into emission between phi=0.20 and phi=0.55. A detailed investigation of the profile variability of the He II lambda4686 emission line reveals that the pattern of variability of this line is very stable. We show that part of the He II lambda4686 emission is produced in the wind interaction region between the stars. Most of the emission lines in the visual spectrum of BD +40 deg4220 display variations that are reminiscent of those observed on the He II lambda4686 line, pointing toward a similar origin. We present numerical simulations of emission-line profiles in a colliding winds binary, showing that an important part of the variability observed in BD +40 deg4220 can be explained by a colliding winds phenomenon. The properties of the wind interaction region can be accounted for if we assume that the secondary star is an evolved object, most probably some kind of Ofpe/WN9 transition star, with a mass loss rate of ~5.5x10^-6 M_solar yr^-1. We finally discuss the fundamental parameters of the binary, concluding that mass transfer must have played a crucial role in the evolution of this system.