  Motivated by recent results by the ATLAS and CMS collaborations on the
angular distribution of the $B \to K^* \mu^+\mu^-$ decay, we perform a
state-of-the-art analysis of rare $B$ meson decays based on the $b \to s \mu
\mu$ transition. Using standard estimates of hadronic uncertainties, we confirm
the presence of a sizable discrepancy between data and SM predictions. We do
not find evidence for a $q^2$ or helicity dependence of the discrepancy. The
data can be consistently described by new physics in the form of a four-fermion
contact interaction $(\bar s \gamma_\alpha P_L b)(\bar \mu \gamma^\alpha \mu)$.
Assuming that the new physics affects decays with muons but not with electrons,
we make predictions for a variety of theoretically clean observables sensitive
to violation of lepton flavour universality.
