More sources:

https://www.flickr.com/photos/estherase/6018055990/
Credit to Esther Simpson - Goggles
https://www.flickr.com/photos/simononly/14562743479/
Credit to Simon Q - Pilot
https://www.flickr.com/photos/ajw1970/20750437711/in/photostream/
Kyle Horner - Tractor's tail
https://www.flickr.com/photos/bannedintheredstates/3458448063/
Alan Wilson - Tractor wings
https://www.flickr.com/photos/jacksnell707/3901294164/
Jack Snell - Propellor

Cool, nice work . Although the plane in the top right and the plane in the top left looks to be partially erased. Maybe you can fix that?

Good one, Looks like threre's a lot of work and imagination, I would suggest you to add an sbs to show the transformations

Pretty funny result, Author! I tend to agree with Skogan, at least for the top right plane (the top left fits better like it's coming from behind a cloud) and I'd put some grain texture over the top right plane too. Ow, before I forget, source 4 is an illustration and I'm afraid it can't be used according to the rules here (eventhough it's offered CC). Is it possible to replace it for a real gun?
Good luck!

Hey guys thanks for your comments. I will fix everything suggested. The top right plane is behind the steam coming out of the tractor's smokestack. The top left plane is behind the smoke trail left by the mid left plane. Guess these aren't defined enough. Will fix, as well as the gun.

Hi. Updated image to show the smoke and steam that the planes were behind. Also replaced the gun.

Will try to do an SBS, but I never have done anything like that before. No promises.