  We construct non-supersymmetric type I string models which correspond to
consistent flat-space solutions of all classical equations of motion. Moreover,
the one-loop vacuum energy is naturally fixed by the size of compact extra
dimensions which, in the two-dimensional case, can be lowered to a fraction of
a millimetre. This class of models has interesting non-abelian gauge groups and
can accommodate chiral fermions. In the large radius limit, supersymmetry is
recovered in the bulk, while D-brane excitations, although non-supersymmetric,
exhibit Fermi-Bose degeneracy at all mass levels. We also give some evidence
for a suppression of higher-loop corrections to the vacuum energy.
