There are so many good choices, but this little guy is probably my favorite thing in the Air Force Museum.
It's a McDonnell XF-85 Goblin. Its job would be to sit in the bomb bay of a B-36, and if/when the bomber was attacked en route to go bomb something, it would open the bomb bay doors and out would come the Goblin to fight off the attacking aircraft. Once the attacking aircraft were taken care of, the Goblin would hook back onto its trapeze in the B-36's bomb bay and the bomber would continue on.
That was the plan, anyway. In reality, it turned out that the air underneath the B-29, which is what they tested the Goblin with, was too turbulent for pilots to reliably be able to hook back onto the bomber, and the program was scrapped when aerial refueling solved the problem by allowing conventional fighter planes to accompany bombers for greater distances.
The yellow landing gear seen in the photograph is not part of the Goblin, as it (theoretically) didn't need landing gear since the plan was that it would hook back onto its bomber. I'm not sure what they did for the emergency landings they had to do during testing when they couldn't get the Goblin to hook back onto the B-29.