Did you use a pencil first? Draw the outlines of the plane and to proportion, then you can start painting. Most of my artwork are pencil sketches but I did paint a few times. You have to learn how to draw first before painting. Use a ruler.
Compare the nose of the picture and your painting. Yours isnt straight.
If you take a ruler and place it on the wings, they line up almost perfectly but at an angle. In your painting, they are not and therefore give a flat look.
The nose and tail are not proportional to each other.
Add some shadows as well, the left horizontal stabilizers should be slightly darker than the right because the rudder is casting its shadow on it. (or vice versa based on where you want your light source to be).
Painting is hard and it takes tons of practice, but for a first attempt, not bad at all, so don't feel bad at the above criticism. Again, remember to draw with a pencil first! You can easily erase a mistake, once you feel that the proportions is right, you have to know where you want your light source to be, visualize the shadows, pick your colors, and use paint strokes that is in the direction of the object (I.e. if you're painting wings, use lateral strokes, if you're painting the rudder, use vertical strokes)