The tech billionaires who for years secretly amassed land in the scrubby grasslands on the fringe of the San Francisco Bay have said they intend to begin gathering signatures for a ballot initiative in Solano County that would clear the way to build a new city from scratch.

The group, called California Forever, envisions a community with tens of thousands of new homes, along with new parks, bike lanes and open space, as well as a solar farm. They are planning a model city. It would be walkable, socioeconomically integrated, and fueled by clean energy.

But to get there, the backers will have to convince Solano County voters. That could be a tough hurdle considering that the project began with secret land purchases and erupted in controversy last year amid unfounded public speculation that the buyers were foreign agents intent on espionage. Federal, state and local approvals will also be required.

Theoretically, backers said, the new city could have a population of several hundred thousand people. But that is a long way off. If the ballot measure is approved, other government approvals will then be required. Lawsuits could also tie up the matter in court. The soonest construction could begin, backers said, is 2026.

“We’re excited to share the details of our plan to build a better Solano with all of you,” Jan Sramek, the chief executive of California Forever, said in a statement. “This can be a new economic engine for Solano County and proof that when we work together, California can still do big things.”

The public campaign is a stark departure from the project’s origins, which began years ago when a mysterious LLC called Flannery Associates began buying thousands of acres of farmland. The purchases, which totaled more than $800 million, made millionaires out of some property sellers, many of whose families had owned the land for more than a century.

But Flannery Associates did not reveal why it was buying the land, which for decades had been devoted to grazing and has more recently been dotted with wind turbines, turning lazily as the Sacramento-San Joaquin Delta flows into the San Francisco Bay.

Because so much of the land is near Travis Air Force Base, some residents and elected officials began to speculate that Flannery Associates was a front for foreign investors seeking to spy on the base, one of the busiest military facilities in the nation.

U.S. Rep. John Garamendi (D-Walnut Grove), who represents the region, said last year that he and other officials had for years been unable to determine who was behind the dizzying land grab and criticized the company for not working with local residents.

“Flannery Associates is using secrecy, bully and mobster tactics to force generational farm families to sell,” the congressman said.

But then the backers were revealed not as spies but as titans of the tech industry, including Reid Hoffman; Marc Andreessen; and Patrick and John Collison, who founded the payment company Stripe.

Now, California Forever is launching a plan to win over those who have expressed skepticism — and there are many. Officials have pledged $400 million in funding for down payment assistance for Solano County residents to buy homes in the new community. They have also unveiled renderings that depict idyllic town squares, tranquil wetlands and charming cafes.

If the measure gains enough signatures, it could go before Solano County voters in November.