USAF had originally designated the plane as the RS-71..but when LBJ was giving a speech, he referred to it as the “SR-71” and that it was, from that day on..
The name change from RS-71 to SR-71 was a last minute decision by Curtis LeMay.
The press briefing packets had it as RS-71, the speech text was changed to SR-71, thus everyone thought Johnson misread it.