To find the volume \( V \) of a cylinder, the formula is:
\[ V = \pi r^2 h \]
where \( r \) is the radius and \( h \) is the height.
In this case, the base radius \( r \) is 8 inches and the height \( h \) is 40 inches.
So, you would fill in the blank with \( 8^2 \):
\[ V = \pi 8^2 \]
To complete the formula you would have:
\[ V = \pi 64 \]
So, the final answer would be:
\[ \pi 64 \]