Having finished third in the constructors' championship for the past two straight seasons, the team are confident their new machine represents a significant step forward - but are in no doubts about the challenge that awaits them.
"Williams has started to cement our position back amongst the front running teams after finishing third in the Championship in the past two seasons," founder and team principal Frank Williams said.
"This has been a great achievement given the resources of those around us. Staying where we are will be a challenge in itself, but we are determined to keep improving because only winning will ever be good enough.
"We have a very stable team going into 2016, with Felipe and Valtteri teaming up for a third successive season. They work well together and both have the ideal blend of speed and consistency.
"Our technical team has also remained very consistent which will be to our advantage and we have some of the best engineering minds on the grid at our disposal."
Deputy team principal Claire Williams added: "2016 promises to be a very competitive year in Formula One and there will be a number of strong teams who have us in their sights.
"We are confident that we have made a step forward over the winter and that the FW38 will build on the strengths of the FW37 and address some of its weaknesses. We started design work on the car earlier than normal and hopefully this extra development time will stand us in good stead.
"The car we are launching today is by no means the final product. We are already planning updates for Melbourne and will be pushing for constant improvements over the course of the season.
"Williams enters 2016 in a strong place. We have a great team of people and over the past two years we have started to regain our competitive edge.
"Now we need to build on that and take this team back to where we all want it to be."
Williams have retained Felipe Massa and Valtteri Bottas for a third straight season - the longest they have kept the same line-up since the days of Ralf Schumacher and Juan Pablo Montoya, whom they fielded from 2001-'04. Bottas finished fifth in the drivers' championship last year, one spot ahead of Massa, with the pair each claiming two podiums during the season.