We have carried out a detailed spectral analysis of six early B-type supergiants in the Sculptor Group spiral galaxy NGC 300. To this end, we used state-of-the-art unified blanketed non-local thermodynamic equilibrium model atmospheres (computed with the code FASTWIND), aimed at the determination of the stellar parameters and of detailed surface abundance patterns. We discuss the individual evolutionary stage of each star in view of their CNO surface abundance, which we compare to massive star evolutionary models with rotation. Although the quantitative comparison does not yield a good agreement, the qualitative behavior is consistent with the model predictions. This issue supports the idea that, although rather evolved, the stars are directly evolving from the main sequence and therefore are in a pre-red supergiant phase. We derive the stellar abundance gradients in the disk of NGC 300 for the elements O, Mg, and Si. Using the available literature data on H II regions in NGC 300, we carry out a detailed comparison of nebular oxygen abundances resulting from different, widely used empirical calibrations. Finally, we compare the abundance gradients derived from the interstellar medium to those derived from the blue supergiants studied in this paper. We find a O/H abundance gradient of -0.033+/-0.026 dex arcmin-1 over a distance equal to the isophotal radius of the galaxy. This trend is also followed in good agreement by Mg and Si abundances. The abundance gradients derived from our stellar data are shallower than those obtained from most previous H II region analyses, and we obtain a lower oxygen abundance in the central region of the galaxy (8.57+/-0.13 dex), which is, however, in agreement with the H II region abundance results derived with recent calibrations of several statistical indicators. Based on observations obtained at the ESO Very Large Telescope.