Very interesting topic. All answers are correct. Mine Are:
3 P-38 Lightning. Howard Hughes designed it originally on a napkin. Highly effective aircraft flown USA leading Ace Dick Bong.
4 B-17 Flying Fortress. Proved daylight formation bombing was practical.
7 F-4 Phantom. Proved if you give a barn door enough thrust it will fly. Aslo first jet aircraft designed to be successfully used by both USAF and USN.
13 B200 King Air. Lead the GA turboprop revolution with variants still in production today.
15 DHC-6 Twin Otter. Designed in the 1960s at the request of the US Arm