OK, time to get (nit)picky. I just want to call out a few things that may not be obvious at first glance. Please consider this as
constructive criticism!
Wing shadows. The main wing's shadows are pretty blurry, but the ones on the tail have sharp edges. I prefer sharp, myself, but I just want to say they should be the same no matter how you decide to make them.
Layering. One of the stripes on the fuselage is partially covering the fuel filler cap under the canopy. They also look like they're covering some panel lines. Like GFR said, don't be afraid to use lots of layers. You also want to make sure they are arranged correctly as well. "Mechanical" bits like the fuel cap should be on a layer above the markings, panel lines, etc. but under the weathering layers.
"Flat" parts. Highlights and shadows are what define your shapes. Be on the lookout for flat looking parts like the exhaust cover, fuselage, wing roots, belly scoop, etc. Don't be afraid to "top shadow" parts to make them appear more rounded. In the specific example of the belly scoop, look at some photos. It will usually look shaded top and bottom with the middle portion in highlight.
Side view
Size. Larger is better. I made the mistake of starting my P-51D profiles at a measly 2500x1400 pixels. The only reason I haven't scaled up to 2 or more times as large is that the amount of work required to redo everything scares me! Still, work at the highest resolution that you are comfortable with. You won't be sorry in the long run.
Can't wait to see more!