The Grove-based outfit had so far only done a season launch with its new livery at an event in New York earlier this month.
And having elected to abandon initial plans for a filming day run at Silverstone this month because of uncertainty over the weather, the team has instead revealed its new FW46 challenger on the day before official pre-season testing begins.
Posts on social media showed the new car in the hands of Logan Sargeant leaving the garage at the Sakhir circuit.
Williams is undergoing a revamp under new owner Dorilton Capital and team principal James Vowles, who was drafted in at the start of last year.
Vowles elected to switch off development of the 2023 car early, so the squad could concentrate more on making the step it needed for this season.
Speaking at the end of last year, Williams head of vehicle performance Dave Robson said that the new car would not look radically different, but would hopefully deliver a good leap in performance.
Asked if it would be evolution or a step change, Robson said: “I think a bit of both. Visibly it will look like an evolution, but I think the subtleties of it are hopefully quite different.
“So things like front locking, some of the balance characteristics, hopefully, we will unlock some big steps now. That is going to make Bahrain testing more challenging just for three days, because I think for sure it’s not going to do exactly what we hoped.
“There’s going to be quite a lot of work to do to maximise it, but hopefully we’ll see some big changes in performance even though visibly it won’t look radical.”
While Williams was encouraged by its seventh-placed finish in last year’s constructors’ championship, the team had to battle with an FW45 car that was tricky in certain conditions.
As Robson said: “I think it was consistently inconsistent. I think we can predict when we were going to struggle, which is a combination of wind, which still hurts us more than more than other teams, in terms of the effect it has on the balance of the car.
“And then there are some corner types, particularly the sort of long, 190 degree corners, if there’s any wind, then you will see all of them in different wind directions as you turn through it. And then the sort of front locking entry into that kind of corner is difficult.”