We present the results of a spectroscopic analysis of 25 galactic luminous OB stars. The gravities are in general lower than previously assumed, especially for giants. The helium abundances are also higher than normal in many cases. Using data from the spectroscopic analysis, radii, luminosities and masses are derived. The observed wind terminal velocities are used with the analytic approximation for the theory of radiatively driven winds (Kudritzki et al., 1989) to obtain masses and mass loss rates. The masses derived from the spectroscopic analysis and those derived independently from the wind theory agree, except for a small difference attributed to deficiencies in the physics of the model atmospheres used. A comparison with evolutionary tracks indicates that (1) evolution theory cannot explain the observed helium abundance pattern and that (2) the masses derived from the spectroscopic analysis (or the wind theory) are systematically lower than those derived from evolutionary calculations.