We apply the Smoothed Particle Inference (SPI) technique to analyze the X-ray emission from SNR W49B. In X-rays, it is visible as a centrally filled supernova remnant, and was recently recognized as one of the first remnants to have plasma that is overionized. Using SPI we infer the density structure and derive the mass of individual elements in the plasma over the entire remnant. We have compared the abundances inferred with SPI to those obtained from a wide range of supernova explosion models, including both Type Ia and core-collapse, as well as energetic and off-center variants. Type Ia models that incorporate some form of detonation are found to be the most compatible, while pure deflagration models, and all core-collapse models, are found to be incompatible.