Non-Standard Trajectory Optimization

A projectile is fired from ground level at an angle of $\theta$ with respect to the horizontal. Let $D$ be the horizontal distance from the initial firing point to the landing point. Let $T$ be the total time the projectile stays in the air.

What value of $\theta$ (in degrees) maximizes the product of $D$ and $T$ ?

Note: Assume an ambient downward gravitational field