Projected rotational velocities (vsini) are presented for a sample of 350 early B-type main-sequence stars in the nearby Galactic disk. The stars are located within ~1.5kpc from the Sun, and the great majority within 700pc. The analysis is based on high-resolution spectra obtained with the MIKE spectrograph on the Magellan Clay 6.5m telescope at the Las Campanas Observatory in Chile. Spectral types were estimated based on relative intensities of some key line absorption ratios and comparisons to synthetic spectra. Effective temperatures were estimated from the reddening-free Q index, and projected rotational velocities were then determined via interpolation on a published grid that correlates the synthetic FWHM of the He_I_ lines at 4026, 4388 and 4471{AA} with vsini. As the sample has been selected solely on the basis of spectral types, it contains a selection of B stars in the field, in clusters, and in OB associations. The vsini distribution obtained for the entire sample is found to be essentially flat for vsini values between 0 and 150km/s, with only a modest peak at low projected rotational velocities. Considering subsamples of stars, there appears to be a gradation in the vsini distribution with the field stars presenting a larger fraction of the slow rotators and the cluster stars distribution showing an excess of stars with vsini between 70 and 130km/s. Furthermore, for a subsample of potential runaway stars we find that the vsini distribution resembles the distribution seen in denser environments, which could suggest that these runaway stars have been subject to dynamical ejection mechanisms.