  While bars are common in disk galaxies, their formation conditions are not
well understood. We use $N$-body simulations to study bar formation and
evolution in isolated galaxies consisting of a stellar disk, a classical bulge,
and a dark halo. We consider 24 galaxy models that are similar to the Milky Way
but differ in the mass and compactness of the classical bulge and halo
concentration. We find that the bar formation requires
$(Q_{T,\text{min}}/1.2)^2 + (\text{CMC}/0.05)^2\lesssim 1$, where
$Q_{T,\text{min}}$ and CMC refers to the minimum value of the Toomre stability
parameter and the central mass concentration, respectively. Bars tend to be
stronger, longer, and rotate slower in galaxies with a less massive and less
compact bulge and halo. All bars formed in our models correspond to slow bars.
A model with the bulge mass of $\sim10$--$20$\% of the disk under a
concentrated halo produces a bar similar to the Milky-Way bar. We discuss our
findings in relation to other bar formation criteria suggested by previous
studies.
