Jarrell was very aware of meter and form in his poetry, so it is a good bet that he didn't write this poem in free verse—lacking any set structure of rhythm or rhyme—just by chance or out of laziness. Likely he felt that following strict metrical patterns or rhyme schemes didn't properly reflect the randomness and chaos of battle. In any case, this poem does contain some interesting metrical moments. For example, take a look at where the stressed syllables fall in line 2:
And I hunched in its belly till my wet fur froze.
The last three words are all single-syllable, stressed words. This gives the end of the line a very strong, almost harsh sound in comparison to the rest of the line, even the rest of the poem—almost like the beating of a drum.
Thinking about this sound along with the line's content, we can see that this metrical element is heightening or emphasizing what is happening in the line: the turret/womb is a cold harsh place in contrast to the mother's womb, and the end of the line sounds harsh, meter-wise.
Another note on the poem's form: When this poem walks into a room, what's the first thing everyone notices? Okay, the fact that it's walking. But let's imagine a world where walking poems are the norm. Then what? You got it. This poem is short. Really short. It is written in one, short stanza. Here again, it would probably be a mistake to chalk this up to laziness on Randall's part.
By making this poem so short, everything (life/death, innocence/war) gets squished together. This also heightens those contrasting elements we've been talking about. For example, by having "mother" so close to "died," the contrasting emotions and ideas in the poem become intensified. Another thing added by this poem's vertically challenged form is a sense that the poem, like the gunner's life, has been cut short.
And you just thought Jarrell wrote a short poem because he wanted you to like him more than all those other long-poem-writing poets.