The former champions finished last in 2018 and have a new driver lineup for this year with Britain's Formula Two champion George Russell making his debut and joined by Poland's Robert Kubica.
Technical director Paddy Lowe said the FW42 car showed promise.
Motorsport.com quoted him as saying after testing in Barcelona that Kubica, who had experience of the old FW41, felt the team had made "a huge step forwards in terms of the platform."
"A car that is far more driveable, a car that you can work with from a driving point of view, you can control your management of tyre, you can control the balance and pace, was definitely not a description we could have given about last year's car," Lowe said.
"So that is very encouraging."
Williams scored just seven points from 21 races last year in one of their worst ever campaigns and travel to Melbourne for the opening race on March 17 with plenty of work to do.
Lowe hoped the team would now have a car that performed as expected.
"Something where you turn into a corner and you know what's going to happen because it happened last time," he said. "This is one of the key aspects that has been improved very considerably."
(Reporting by Alan Baldwin; Editing by Andrew Cawthorne)
