Williams rookie Valtteri Bottas believes that the FW35 is an improvement on the FW34.
Bottas drove the car for the first time in Spain during testing on Wednesday, setting the sixth fastest lap time.
"I had a really strong first impression of the FW35 today," said Bottas.
"Once we completed the aero work this morning I was able to complete my first proper run to feel the FW35 and the improvements we have made."
The 2011 GP3 champion completed almost 100 laps at the Circuit de Catalunya.
"Straight out of the box I knew we had made a step forward from last year, and I am looking forward to working to develop the car even further. It’s great to know we have a really good base to start from but there are still a number of areas we can improve, so I’m really excited for the season ahead."
Bottas and team-mate Pastor Maldonado will share the running across the remaining two days.
