The formula for the surface area \( A \) of a sphere is given by:
\[ A = 4\pi r^2 \]
where \( r \) is the radius of the sphere. The diameter of the sphere is 40 inches, so the radius \( r \) is half of that:
\[ r = \frac{40}{2} = 20 \text{ inches} \]
Now, we can substitute the radius into the surface area formula:
\[ A = 4\pi (20)^2 \]
Calculating \( (20)^2 \):
\[ (20)^2 = 400 \]
Now substituting this value back into the surface area formula:
\[ A = 4\pi (400) \]
\[ A = 1600\pi \]
Thus, the surface area of the sphere is
\[ \boxed{1600\pi} \text{ square inches.} \]
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