Consider the following partial differential equation:

$$3 \frac{\partial ^2 \phi}{ \partial x^2} + B \frac{ \partial ^2 \phi}{\partial x \partial y} + 3 \frac{\partial ^2 \phi}{\partial y^2} + 4 \phi =0$$

For this equation to be classified as parabolic, the value of $B^2$ must be ________