  We consider the Einstein equations within the DBI scenario for a spatially
flat Friedmann-Robertson-Walker (FRW) spacetime without a cosmological
constant. We derive the inflationary scenario by applying the symmetry
transformations which preserve the form of the Friedmann and conservation
equations. These form invariance transformations generate a symmetry group
parametrized by the Lorentz factor $\ga$. We explicitly obtain an inflationary
scenario by the cooperative effect of adding energy density into the Friedmann
equation. For the case of a constant Lorentz factor, and under the slow roll
assumption, we find the transformation rules for the scalar and tensor power
spectra of perturbations as well as their ratio under the action of the form
invariance symmetry group. Within this case and due to its relevance for the
inflationary paradigm, we find the general solution of the dynamical equations
for a DBI field driven by an exponential potential and show a broad set of
inflationary solutions. The general solution can be split into three subsets
and all these behave asymptotically as a power law solution at early and at
late times.
