No, we’re not talking about stealth fighters — Grumman’s invisible jet fighter was an example of a very early flight simulator. Using a Reeves analog computer, Grumman compiled test data using models and simulations to program the computer and ‘flight-test’ imaginary aircraft’s data against actual flight information. Analog simulators had been around for about a decade, and digital simulators were gaining a foothold (although, as the advertisement says, Grumman didn’t think much of digital simulation). Analog eventually gave way to real-time digital simulation with the Navy’s UDOFT, the Universal Digital Operational Flight Trainer, which started shortly before Grumman’s ad but wasn’t completed until 1960. As you might guess, these projects eventually spawned imitators in the private sector, resulting in grainy, wireframe simulators — that ran on small personal computers.