  In this paper, we explicitly construct string theory backgrounds that realise
the so-called $\mathcal N=2^\star$ gauge theory. We prove the consistency of
our models by calculating their partition function and obtaining the correct
gauge theory spectrum. We further provide arguments in favour of the
universality of our construction which covers a wide class of models all of
which engineer the same gauge theory. We reproduce the corresponding Nekrasov
partition function once the $\Omega$-deformation is included and the
appropriate field theory limit taken. This is achieved by calculating the
topological amplitudes $F_g$ in the string models. In addition to heterotic and
type II constructions, we also realise the mass deformation in type I theory,
thus leading to a natural way of uplifting the result to the instanton sector.
