Exceptionally well rendered for a piece that uses no reference material and was done in 4-5 hours. The composition itself has a good balance of light and darks as far as positioning and tonal depth with the lighter tones in the upper and upper/right sections balanced by your deeper shadows and greys of the left and lower/left in the details of the suit and shadows of the building.
The rough quality of the painting itself does help give off that gritty/war sort of erratic energy but it also seems to make the picture a bit blurry in some areas rather than energy-charged. Since the buildings in the far back plane of the composition are almost unrecognizably distorted I'm assuming your going for a semi-depth-of-field effect in which case I would recommend going back and touching up the foreground of the machine adding more details here and there, especially around the,...I guess I would call it a fender, that covers the wheels (i think they're wheels and not tank tracks) and refine that up a bit since the machine seems very bland next to the much more detailed figure standing infront of it which throws it off since they are the same distance from the picture plane.
Personally I'd like to see the machine sharpened up just a bit and maybe add some texture to it like big dents and bullet impact spots riddling it. Also you might adjust the color of the sky as I think it takes away from the visual "bang" of the machine because they are in a very similar color range, or possibly colorize the machine and give it sort of a chipped/scratched paint look overall, though I understand the tan finish as it is seen on alot of desert combat tanks and artillery. Just as my own opinion I think a night or dusk version of this composition would be exceptionally dramatic as you could possibly have the guns firing with large muzzle flashes and onboard lights glowing. All in all an excellent example of imagination on the loose. Great work.