"I'm confident that we'll be closer to the front aerodynamically than we were last year," said engineering head Pat Symonds in a statement.
The new FW36, in a blue and unbranded temporary livery, revealed the new slimline 'anteater-style' nose expected to be seen on most cars this year after regulation changes imposed for safety reasons.
Williams, who scored just five points last year and finished ninth overall, are the first of the teams to show their nose design - even if it will be changed for the first race in Australia on March 16.
Formula One is ditching the old V8 engine to a turbocharged V6 with energy recovery systems in the biggest technical upheaval for a generation of engineers.
Symonds said the new partnership was already delivering strong results.
"This is the first time that Williams has worked with Mercedes in F1 and we've been very impressed," said the former Renault technical director. "We're as confident as we can be that the power unit will be competitive."
Williams will also use an eight speed gearbox for the first time this season.
"It's completed a lot of running on the test rig...but you can't take reliability for granted," said Symonds. "It has to cope with a lot more torque than was the case with the V8."
The team said the new car had passed all the mandatory crash tests before Christmas and they would be at the first test starting in Jerez, southern Spain, next week.
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