Controversial plans to transform over 50,000 acres of farmland into a city in Solano county, northern California, have been unveiled by a group of investors named Flannery Associates.

In a collection of colourful illustrations, the city is depicted as a series of walkable neighbourhoods that the group said will offer "a chance for a new community".

However, the plans have sparked widespread criticism from locals over Flannery Associates's quiet buying up of farmland, which has even drawn attention from the FBI due to its proximity to an air force base, according to newspaper The Guardian.

The plans for the city were revealed by the New York Times last week, which also confirmed that Flannery Associates was formed of prominent Silicon Valley investors.

Since the New York Times article, Flannery Associates has launched a website to outline its vision – California Forever.

According to California Forever, the aim of the city is to create walkable neighbourhoods that will be powered by clean energy, have a reliable public transport network and create thousands of jobs. It imagines homes of various sizes that sit in close proximity to shops, schools and open spaces.

The unveiling follows mounting criticism in recent years while the group was quietly buying up the farmland, eventually becoming "the largest landowner in the county".

"To date, our company has been quiet about our activities," Flannery Associates said on the California Forever website.

"This has, understandably, created interest, concern, and speculation. Now that we're no longer limited by confidentiality, we are eager to begin a conversation about the future of Solano county," it continued.

According to The Guardian, the land purchased for the scheme comprises a mix of agricultural plots and empty plots in Solano county.

Details of the plans such as its specific location or projected population are so far limited, but the website states that the proposal will not impact "the zoning of other landowners' properties".

However, Flannery Associates said it will be working with the local community to develop it.

"Our team is working closely with the community and will continue to meet with local leaders to craft a shared vision for Solano county's future," it wrote.

It also claims that it is working with a team of experts, such as architecture practice SITELAB Urban Studio, communications agency AKQA and engineering firm Carlson, Barbee & Gibson.

Since Thursday, the news has ignited further uproar from both officials and residents living in the county, according to reports.

Among them is state senator Bill Dodd, who referred to the backers as "a shadowy investment group".

"People in my district are understandably alarmed," he said in a statement. "But we don't really know what's going on because the investors have not shared anything with locals."

Meanwhile, in conversation with the Daily Beast, the mayor of Fairfield Catherine Moy said that locals were developing "a plan for defence".

"It's not our first rodeo," Moy explained. "The one thing that I'll say Flannery has done is they brought us together," she continued.

"And by that I mean, citizens, environmentalists, other builders, politicians – all together to build a wall to stop Flannery."

Another controversial city that is currently under development is The Line, a 170-kilometre-long, 500-metre-tall and 200-metre-wide metropolis planned for Saudi Arabia.

Other recent city masterplans for the US include Telosa by BIG and Innovation Park by Ehrlich Yanai Rhee Chaney Architects and Tom Wiscombe Architecture.

The illustrations are courtesy of California Forever.