To simplify the expression \( D = (2x + y)^2 - (2x - y)^2 \), we can use the difference of squares formula, which states that \( a^2 - b^2 = (a - b)(a + b) \).

Let:
- \( a = 2x + y \)
- \( b = 2x - y \)

Now, applying the difference of squares:

\[
D = (a - b)(a + b)
\]

Calculating \( a - b \) and \( a + b \):

1. **Calculate \( a - b \):**
\[
a - b = (2x + y) - (2x - y) = 2y
\]

2. **Calculate \( a + b \):**
\[
a + b = (2x + y) + (2x - y) = 4x
\]

Now substitute these results back into the formula for \( D \):

\[
D = (2y)(4x) = 8xy
\]

So the simplified result is:

\[
D = 8xy
\]