  In the framework of the study of supernova remnants and their complex
interaction with the interstellar medium, we report on an XMM-Newton EPIC
observation of the Galactic supernova remnant W49B. We investigate the spatial
distribution of the chemical and physical properties of the plasma, so as to
get important constraints on the physical scenario, on the dynamics of the
supernova explosion, and on the interaction of the supernova remnant with the
ambient interstellar clouds. We present line images, equivalent width maps, and
a spatially resolved spectral analysis of a set of homogeneous regions. The
X-ray spectrum of W49B is characterized by strong K emission lines from Si, S,
Ar, Ca and Fe. In all the regions studied, the X-ray spectrum is dominated by
the emission from the ejecta and there is no indication of radial
stratification of the elements. A high overabundance of Ni
(Ni/Ni_sun=10^{+2}_{-1}) is required in the bright central region and the
previous detection of Cr and Mn line emission is confirmed. Spectra are well
described by two thermal components in collisional ionization equilibrium. We
observe spatial variations in the temperature, with the highest temperature
found in the east and the lowest in the west. Our results support a scenario in
which the X-ray emission comes from ejecta interacting with a dense belt of
ambient material, but another possibility is that the remnant is the result of
an asymmetric bipolar explosion with the eastern jet being hotter and more
Fe-rich than the western jet. The eastern jet is confined by interaction with
ambient molecular clouds. Comparison of the observed abundances with yields for
hypernova and supernova nucleosynthesis does not directly support the
association of W49B with a gamma-ray burst, although it remains possible.
