I've always been partial to the idea of bombers, since that's is typically how nukes were envisioned to be dropped until the missile threats of the 60s when FAR more accurate missiles were developed. Plus I believe one of the intro slides of Fallout 1 showed a bomber, IIRC. Rockets/missiles of the fiction period Fallout borrows from were often unpredictable and kinda useless except as an intimidation tool, notably the German V2 launches. So...I think we could consider the installation of The Microchip (which never existed in Fallout) into the Minuteman as the defining moment when ICBMs were a real threat. As for Fallout canon, it's kind of a mixed thing, given parts of Fallout 2 and elsewhere. Most missiles of that time were from the Nazi's buzz bombs to other experiments and went onto the modern joke of the Scud (horrible guidance system for it's comparatively small range), the Matador wasn't that much better, but it wasn't until the Minuteman would a missile really be accurate enough to pinpoint targets with the precision they would want of a nuke. The topic of ICBMs really didn't hit public imagination wildly until the 60's, naturally, with the Cuban Missile Crisis and the events leading up to it.