Visual Aviation Quiz

I am a fan of aviation and quizzes, so here is a combination of both.  I think that it is interesting, because the information you need is here. I don't think that Google or Wikipedia will help.

What do these four airplanes have in common?

1. Curtiss P-40 Warhawk

2. Douglas DC-3

3. Fairchild A-10 Thunderbolt II AKA Warthog

  

4. Boeing 737

 
Hints:  Here are some things the planes don't have in common:

• Military or civilian - there are 2 of each
• Engine count: 1 has a single engine, while the other 3 are twins
• Engine type: 2 are piston-driven, 2 are jets
• Nose wheel: two are "tail-draggers" while two have "tricycle" gear
• Tail type: 1 has twin rudders, the others have single rudders
• Manufacturer: all four are different
• Vintage: 2 are from the 30s, 1 from the 60s, 1 from the 70s

Big Hint: When you look at the pictures, look down.

Answer: Please see this post.