  We measure the kinematic Sunyaev-Zel'dovich (kSZ) effect, imprinted by maxBCG
clusters, on the Planck SMICA map of the Cosmic Microwave Background (CMB). Our
measurement, for the first time, directly accounts for uncertainties in the
velocity reconstruction step through the process of Bayesian forward modeling.
We show that this often neglected uncertainty budget typically increases the
final uncertainty on the measured kSZ signal amplitude by $\simeq15\%$ at
cluster scale. We observe evidence for the kSZ effect, at a significance of
$\simeq2\sigma$. Our analysis, when applied to future higher-resolution CMB
data, together with minor improvements in map-filtering and signal-modeling
methods, should yield both significant and unbiased measurements of the kSZ
signal, which can then be used to probe and constrain baryonic content of
galaxy clusters and galaxy groups.
