We give an explanation for the signal detected by NANOGrav as the stochastic gravitational wave background from binary mergers of primordial Stupendously Large Black Holes (SLABs) of mass $Msim(10^{11}-10^{12})M_{odot}$, and corresponding to roughly $0.1%$ of the dark matter. We show that the stringent bounds coming from $mu$ distortions of the CMB can be surpassed if the perturbations resulting in these BHs arise from the non-Gaussian distribution of fluctuations expected in single field models of inflation generating a spike in the power spectrum. While the tail of the stochastic background coming from binaries with $Mlesssim 10^{11}M_{odot}$ could both fit NANOGrav and respect $mu$ distortions limits, they become excluded from large scale structure constraints.