I’m somewhat confused. Are we supposed to only use the techniques we’re learning in these lessons? I went with only extrudes, scales, etc, and didn’t use any additional shapes to add to it. I spent literally hours trying to decide something I could model which took forever also. 
I originally wanted to do a biplane, but couldn’t figure out the dimensions to make my tiny boxes ahead of time for the multiple direction extrusions I would need.
I don’t think there are any limitations. The practice models are just that - use your skills and all the techniques you have available to create something.
Your plane reminds me of the stealth bomber in the middle. It seems futuristic or alien!