  The origin of the asymmetric supernova remnant (SNR) W49B has been a matter
of debate: is it produced by a rare jet-driven core-collapse supernova, or by a
normal supernova that is strongly shaped by its dense environment? Aiming to
uncover the explosion mechanism and origin of the asymmetric, centrally filled
X-ray morphology of W49B, we have performed spatially resolved X-ray
spectroscopy and a search for potential point sources. We report new candidate
point sources inside W49B. The Chandra X-ray spectra from W49B are
well-characterized by two-temperature gas components ($\sim 0.27$ keV +
0.6--2.2 keV). The hot component gas shows a large temperature gradient from
the northeast to the southwest and is over-ionized in most regions with
recombination timescales of 1--$10\times 10^{11}$ cm$^{-3}$ s. The Fe element
shows strong lateral distribution in the SNR east, while the distribution of
Si, S, Ar, Ca is relatively smooth and nearly axially symmetric. Asymmetric
Type-Ia explosion of a Chandrasekhar-mass white dwarf well-explains the
abundance ratios and metal distribution of W49B, whereas a jet-driven explosion
and normal core-collapse models fail to describe the abundance ratios and large
masses of iron-group elements. A model based on a multi-spot ignition of the
white dwarf can explain the observed high $M_{\rm Mn}/M_{\rm Cr}$ value
(0.8--2.2). The bar-like morphology is mainly due to a density enhancement in
the center, given the good spatial correlation between gas density and X-ray
brightness. The recombination ages and the Sedov age consistently suggest a
revised SNR age of 5--6 kyr. This study suggests that despite the presence of
candidate point sources projected within the boundary of this SNR, W49B is
likely a Type-Ia SNR, which suggests that Type-Ia supernovae can also result in
mixed-morphology SNRs.
