Double-klick on the integrator blocks and you'll see that the default initial condition is zero (0). It is also inside the integrator block where you change the intial condition, or, select that it is defined through an external input.
Also, I noticed in your model that the feedback loop starts after the second integrator, while your differential equation contains a Xdot. To make the Simulink model consistent with the equation the loop would have to start between the first and second integrator.
