I was searched with one around the year 2000, IIRC, at the San Francisco airport. I think it was pretty experimental.

edit: there might have been a similar setup at Orly in France in 1997, come to think of it. The striking thing there was the soldiers with rifles (just 3 of them, but walking around) and the way they searched one of my grandmother's bags when the x-ray showed something that looked like a grenade. (it was glass in a weird shape) They were friendly and had a sense of humor about it, and it didn't slow us down, which all made a big difference.

Most European airports have had those as long as I can remember (definitely since the mid 90s).

Maybe bomb detectors but the explosive detectors ('puffers') are relatively new as far as I know.

It was a huge political budget fight in the 90s. Clinton/Gore published a white paper detailing the threat of terrorism as the biggest threat to national security. They called for the fancy new detectors and what-not at airports.

The Republicans, who controlled Congress, said that the big threat was from nukes from unfriendly states (later dubbed 'the axis of evil') didn't permit the funding and instead increased 'Star Wars' funding (anti-missile tech).