The FW35 will be driven by Pastor Maldonado and Finnish rookie Valtteri Bottas.
Williams opted to use the FW34 at the first pre-season test at Jerez in order to undertake comparisons between the 2012 and 2013 model.
The FW35 features 80% new parts, with 'a new gearbox, new rear suspension, new radiators, a new floor, new exhausts, new bodywork, a new nose'.
"Starting my third season with Williams is important for both myself and the team," says Maldonado.
"We’ve been working hard to develop the car and improve our performance. Last year we won a race and were competitive, but this season we must be even more competitive which I believe is possible."
"Consistency will be our aim to close the gap to the teams in front. There are a number of changes this season; I have a new Race Engineer and a new teammate in Valtteri, but there is a nice atmosphere right now as we are all competitive and know each other well. Being part of the Williams family is very special as a driver and I want to thank Frank for not only giving me the opportunity to drive for him, but also to deliver a win. My goal is to help take the team back to the top."
Bottas adds that he feels ready for his first season in the sport.
"It feels great to now be promoted to race driver with Williams this season".
"It’s a big step and I’m really looking forward to it. I have been doing a lot of training, meeting with my engineers and working in the simulator in preparation; so I feel ready. We know we have improved our car from last season but I am looking forward to seeing how it compares on track. We have a busy test programme ahead but I’m really excited. The moment the start lights go out in Melbourne is a moment I have been building towards my whole career."
The FW35 will make its track début today as the second test begins at the Circuit de Catalunya.