Well now, I'm sure the US designation system is full of chaotic and confusing things... however, the fact that it contains lots of exceptions doesn't mean that it doesn't follow a set of rules
. For each exception, there's usually a more or less sensible explanation. In particular, when it comes to the Roman numerals. Yes, they're hugely inconsistent. But again, that's because names are supposed to be evocative, not encyclopedical. The Thunderbolt II doesn't carry a II to remind people that it's the second Thunderbolt out there, it carries a II to suggest to people that this plane is going to be as awesome as the first Thunderbolt. It's more about marketing than logic or accuracy.
I'll give you an example far crazier than any of the ones you pointed out: the Corsair. There were no less than four different Corsairs, (O2U: 1926; SBU: 1934; F4U: 1940; then a long break, and the A-7 arrives in 1963) all produced by the same manufacturer, and three of them named Corsair, while only the last one was Corsair II. Crazy, isn't it? Well, it's not, if you think about how the first two Corsairs were totally unknown (they're interwar planes, never had a chance to see any combat in the US Navy, though the O2U Corsair actually did see quite a lot of action elsewhere in the world), and how the third Corsair became really, really famous, it makes sense that the first two were overlooked, and it's only with the fourth Corsair that a reference to a previous Corsair was deemed worthwhile. Again, you want the name to make pilots feel awesome about flying the plane, and the public to feel awesome about paying for the plane. That being considered, you probably wouldn't want to remind them that hey, this happens to be the third plane named Corsair, and the previous two were utterly forgettable
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Incidentally, it seems that the US Navy didn't really name planes in the 1920s and 1930s. I'm not sure when they did start naming them, but up to a certain point in time, it seems that it was the manufacturers who came up with names (so, yeah, Vought really liked "Corsair"
). This also means that some planes had designations only, because the manufacturer did not give them a name. I'm only mentioning this because we're talking about Vought, whose letter code in the USN designation system was "U". Consequently, the very first Vought fighter to enter Navy service was entirely nameless, but carried an utterly awesome designation: FU.
"So, you're a fighter pilot, Lieutenant? What kind of aircraft do you fly?"
"FU, Admiral..."