  We present a comprehensive X-ray spectroscopy and imaging study of supernova
remnant W49B using archival XMM-Newton observations. The overionization state
of the shocked ejecta in W49B is clearly indicated by the radiative
recombination continua of Si XIV, S XV, and Fe XXV, combined with the
Ly$\alpha$ lines of Ca and Fe. The line flux images of W49B indicate high
emission measures of the central bar-like region for almost all the emission
lines, while the equivalent width maps reveal a stratified structure for the
metal abundance distributions. The global spectrum of W49B is well reproduced
by a model containing one collisional ionization equilibrium (CIE) plasma
component and two recombining plasma (RP) components. The CIE plasma represents
the shocked interstellar medium, which dominates the X-ray emitting volume in
W49B with a mass $\sim450M_\odot$. The two RP components with a total mass
$\sim4.6M_\odot$ are both dominated by the ejecta material, but characterized
by different electron temperatures ($\sim1.60$ keV and $\sim0.64$ keV). The
recombination ages of the RP components are estimated as $\sim6000$ yr and
$\sim3400$ yr, respectively. We then reveal the possibility of a thermal
conduction origin for the high-temperature RP in W49B by calculating the
conduction timescale. The metal abundance ratios of the ejecta in W49B are
roughly consistent with a core-collapse explosion model with a
$\lesssim15M_\odot$ progenitor, except for a rather high Mn/Fe. A Type Ia
origin can explain the Mn abundance, while it predicts much higher ejecta
masses than observed values for all the metal species considered in our
analysis.
