A non-LTE model atmosphere analysis of moderate resolution (B similar to 5000) spectra of 36 Galactic B-type supergiants is presented. Standard techniques are adopted, viz. plane-parallel geometry and radiative and hydrostatic equilibrium. Spectroscopic atmospheric parameters (T-eff, log g & upsilon(turb)) and chemical abundances (He, C, N, O, Mg & Si) are estimated, both as a test of the validity of such an approach and in an attempt to provide consistent results for supergiants covering a significant range of spectral types. The values of the estimated atmospheric parameters and their dependence on the physics adopted in the model atmospheres calculations are discussed. The absolute metal abundances are compared to those of main sequence B-type stars and, in general, their chemical compositions appear to be similar. The abundances for He, C, N & O are considered in some detail and are discussed in the context of possible evolutionary histories for this stellar sample. Specifically, it is found that the supergiant sample can be subdivided into a number of evolutionarily distinct groups. The lower mass objects are predominantly chemically near-normal i.e. their photospheres show little or no evidence for chemical processing, whereas the higher mass supergiants have CNO ratios which are indicative of CN and possibly NO-cycle burning. An attempt is made to quantify the difference in nitrogen and carbon abundances between the high and low mass targets but this is hampered by theoretical uncertainties. The possibilities that the most highly processed supergiants may have either larger rotational velocities or have undergone mass transfer within a binary system are discussed.
