I’m surprised nobody mentioned the B-58 (probably because I’m wrong about it never seeing combat). Our first operational supersonic bomber. It had that screwy detachable bomb bay (that looks like the external fuel tanks sometimes carried by fighter aircraft).
As with the B-47, the closest the B-50 saw to combat was reconnaissance, but it should be noted that those pre U-2 early Cold War reconnaissance missions were VERY close to combat. Both the RB-50’s and RB-47’s were armed with turrets and shot it out with MiG’s from the Soviet air defense on multiple occasions, with several examples of both planes were shot down over the USSR.
No, but you’re wrong about nobody having mentioned it. See posts #16 and #17.
Curtiss F11C Goshawk
Curtiss SBC Helldiver
Martin B-10
[Keystone B-6](Keystone B-6)
Curtiss Falcon
among quite a few others from pre-WWII.
Both too late for WWII, but obsolete immediately afterwards.
Although it never made it past the experimental stage, the Goblin might be a bit cuter.
Interesting topic. Was the “goblin” fighter ever used? It was a tiny little plane, that could be launched from a bomber.
Perhaps I should have limited the discussion from 1938 onwards. Prior to WWII, developments were coming quickly and were soon superseded. The scale of the developments, for example, from tube-and-fabric biplanes to all-aluminum monoplanes, retractable landing gear, and so on, ensured that some types accepted into service were never used in combat. This also happened, as posted by others here, in the 1950s; but my impression is that the years between the World Wars were a time when isolationism was quite strong in this country. Those sentiments would also limit combat usage.
Besides, my knowledge of aircraft in the 1920s and early-1930s is somewhat lacking. :o 
There are some interesting planes mentioned here. B-58, B-36, the ‘Gutless Cutlass’… Quite reasonable that they were never used in combat. The F-106, though, I would have thought was used in Vietnam. The F-102s were for a little while. I also thought the F-101 was used as a bomber there. The F-22… It’s still in service, so there’s time.
As far as I can tell, these never saw combat:
Republic F-84F Thunderstreak
North American FJ-2/3 Fury (not sure if navalized versions count as different planes)
North American FJ-4 Fury (same naming scheme, different plane)
McDonnnell F3H Demon
I just watched a program about parasite aircraft. The 2 Goblin prototypes only made 7 flights in total, and only 3 ended with successful dockings.
No helicopters?
Helicopters started out as reconnaissance/search/rescue machines, so those wouldn’t count for the purposes of this thread. Certainly, many types were adapted to attack roles; but that’s not what the were designed for. U.S. dedicated attack helicopters seem to have been built as prototypes and not put into production (e.g., AH-56 Cheyenne), or else they were put into production and used in combat (e.g., AH-1 series).
[Aside]
Some military helicopters were militarised versions of civilian machines; or they were designed for the military and then ‘civilised’:
H-13 Sioux = Bell 47
TH-55 Osage = Hughes 269/Schweizer 300/Sikorsky S.300
OH-58 Kiowa = Bell 206 JetRanger/Bell 207 LongRanger
UH-1 Iroquois = Bell 204/Bell 205/Bell 212
Attack helicopters… Not so much.
[/Aside]
Fixed-wing aircraft started out as reconnaissance machines too. We’ve had attack helicopters since the 60s so there must have been a couple that never saw action.
Yes, fixed-wing aircraft started out as reconnaissance machines; but they quickly began being designed specifically for various combat roles such as air-to-air and air-to-ground combat. For example, the P-51 was not designed as a recon plane, and the B-52 was not designed as a cargo plane.
As for helicopters designed for attack, I can’t think of any that went into production that didn’t go into combat. The AH-56 came close, but it was cancelled. But then, the only U.S. attack helicopters I can think of are the AH-1 Cobra series and the AH-64 Apache.