*area rule reduces drag by smoothing the rate of change of the longitudinal cross section area of the total airplane. To do this the belly fairing was carefully shaped and the fuselage is concave at the pylons.

The X3 after the X1 X1a and X2 was to go Mach 3. It couldn't even break mach 1 because of Area rule.
The coke bottle shape, or the fuselage is concave to draw the shock wave closer in to the wing root.