As we mentioned in our "Speaker" section, the gunner is speaking to us from beyond the grave. But most of the action he describes takes place in and around a WWII bomber. More specifically, in the bomber's, "belly."
This setting is crucial for the poem because it develops the birth imagery that the poem relies on. Without the turret/womb, without the setting, this poem would lose some of its punch. The poem relies on this birth imagery to show the futility of the state as nurturer or protector. The poem would lose the sense of the machine-womb birthing the soldier into death rather than life.
The setting is what creates the feeling of tension and conflict and confusion. It mirrors a place that is supposed to be all warm and nurturing, but the bomber version of Mommy is far from nurturing. A womb should be a warm, safe place that holds the anticipation of birth and life. But the turret/ womb is the opposite: cold, dangerous, and with only the anticipation of death.
Imagine the poem set in the cockpit, at the plane's head rather than in its belly. Different poem, right? The pilot, sitting in the cockpit, has some sense of control. He can change directions. He can order the crew to bail out. The gunner, though, is stuck in the turret. He has no control. Fighting and dying are pretty much all turret gunner has left.
Jarrell wants us to feel war at a very base, gut level. This poem is not cerebral. It's not about the causes of war or what can be done to find peace. It is about life and death and the brutality of war. The belly is where this poem has to happen.