  We investigate D-brane instanton contributions to R^4 couplings in any
toroidal compactification of type II theories. Starting from the 11D
supergravity one-loop four-graviton amplitude computed by Green, Gutperle and
Vanhove, we derive the non-perturbative O(e^{-1/\lambda}) corrections to R^4
couplings by a sequence of T-dualities, and interpret them as precise
configurations of bound states of D-branes wrapping cycles of the
compactification torus. Dp-branes explicitely appear as fluxes on
D(p+2)-branes, and as gauge instantons on D(p+4)-branes. Specific rules for
weighting these contributions are obtained, which should carry over to more
general situations. Furthermore, it is shown that U-duality in D<=6 relates
these D-brane configurations to O(e^{-1/\lambda^2}) instantons for which a
geometric interpretation is still lacking.
