Oh come on! These bombers would fall apart after that time
If you ask me, they used advanced high-tech superbombers,
which is still fine with me if you assume they did not have sufficiently developed rocket technology , though I am oblivious as to why they shouldn't. Also, , I'm not an expert on this one, but in the game, Maxson's diary said "they've let the A-bomb fly" so at first, I imagine he was referring to the nuclear misslies (or rocket propelled A-bombs if you like it better
)
And the deal with the bombers in the intro is IMHO simple - they weren't meant to depict the actual the machines used to bomb USA, they were just supposed to build up climate. Remember that Ron makes some historical references to past wars in the intro. I mean, on the game authors' side, it was easier to use photos instead of rendering some hyper-advanced air fleet , which could not contribute to the game's "feel" as good as the pictures actually used.