  Low resolution (4.5 to 5 Angstroem) spectra of 58 blue supergiant stars
distributed over the disk of the Magellanic spiral galaxy NGC 55 in the
Sculptor group are analyzed by means of non-LTE techniques to determine stellar
temperatures, gravities and metallicities (from iron peak and alpha-elements).
A metallicity gradient of -0.22 +/- 0.06$ dex/R_25 is detected. The central
metallicity on a logarithmic scale relative to the Sun is [Z] = -0.37 +\- 0.03.
A chemical evolution model using the observed distribution of stellar and
interstellar medium gas mass column densities reproduces the observed
metallicity distribution well and reveals a recent history of strong galactic
mass accretion and wind outflows with accretion and mass-loss rates of the
order of the star formation rate. There is an indication of spatial
inhomogeneity in metallicity. In addition, the relatively high central
metallicity of the disk confirms that two extra-planar metal poor HII regions
detected in previous work 1.13 to 2.22 kpc above the galactic plane are ionized
by massive stars formed in-situ outside the disk. For a sub-sample of
supergiants, for which Hubble Space Telescope photometry is available, the
flux-weighted gravity--luminosity relationship is used to determine a distance
modulus of 26.85 +\- 0.10 mag.
