What if you built a new city from the ground up? The idea has tantalized urban planners and utopian dreamers for centuries. In 2017, Jan Sramek joined their crusade.

Mr. Sramek, a former Goldman Sachs trader, had moved to the San Francisco Bay Area to make it in tech. He was a European immigrant smitten with the energy of local start-ups, but he preferred more walkable cities like Zurich. Soon, he began taking fishing trips to Solano County on the San Francisco Bay’s eastern edge.

A rural corner of the county eventually became the centerpiece of a plan hatched by Mr. Sramek to build a city from scratch. He created a company called Flannery Associates, and has spent the last few years using money from some of the wealthiest people in Silicon Valley to make his audacious idea come true, said two people with knowledge of his work who were not authorized to speak publicly.

The story of how Mr. Sramek got some of the richest people in the world to buy $900 million in farms and undeveloped land with the dream of a new city seems destined to become a Silicon Valley legend that mixes idealism — or hubris — with old-fashioned capitalism.

Until last week, when The New York Times revealed the company’s investors and plans, no one in the county had any idea who was behind Flannery. As the company gobbled up land, suspicions about its identity and intentions escalated from Facebook posts to county supervisors to a national security scare that prompted the F.B.I. and the Treasury Department to investigate.

The notion of creating a city where the cars are autonomous and the regulation is light had been bouncing around the meetings and salon parties of Silicon Valley’s tech elite for years. But Mr. Sramek had a specific plan for how investors could acquire the property, said the two people familiar with his plans.

After taking the idea to a dozen potential funders, he got his first check from Patrick Collison, the chief executive of Stripe, a payments company with a rising valuation that had Mr. Collison made a billionaire on paper.

Within a few months, those conversations grew into Flannery Associates. What began with the relatively modest goal of buying 10,000 acres is now a full-blown land grab, according to court documents and an early investor pitch reviewed by The Times. The company has ballooned into a $900 million effort that has made it the owner of nearly enough parcels to cover San Francisco twice.

Now that the company and its intentions are public, Flannery is expected to spend the next several months trying to charm elected representatives and rally voters around its plan. It could be years — if ever — before the company gains approvals from local and state officials and any real work is started.

On Thursday evening, Flannery unveiled a website explaining its plans. It said Flannery had a parent company, California Forever that had spent the past few years polling residents about what they wanted to see. The website noted that in regional plans drawn up decades ago, local and federal planners had identified eastern Solano County as a site for potential development. It predicted a “decades-long collaboration” with residents and the local government.

“I can’t imagine the supervisors allowing such a thing,” said Robert McConnell, the mayor of Vallejo, referring to Solano County’s Board of Supervisors. Vallejo, population 126,000, is the largest city in the largely agricultural county.

“I’m surprised that people that intelligent would waste their time and money and effort on this,” he added.

Until last week, Mr. Sramek was known mostly for a job he quit. In 2011, two years after being branded a 22-year-old golden boy who named the billionaire venture capitalist Peter Thiel as a role model, he left Goldman Sachs and expressed dreams of founding a company.

His first company was an education software provider called Better. He moved to the Bay Area, where many of its customers were, and sold the company in 2015. After that came Memo, a social media service for ideas and learning that never caught on. In a blog post, Mr. Sramek and his co-founder, Carl Baatz, blamed the modern world’s lowbrow tastes for Memo’s failure.

Around that time, a mutual friend connected Mr. Sramek to Mr. Collison, a wide and voracious reader whose payments company was pushed into more esoteric areas like book publishing and carbon removal as it became one of the most valuable private companies in tech.

Mr. Collison was also part of the budding movement of young people who advocated looser development laws to make it easier to build housing. Along with his brother, John Collison, he donated $1 million to the California YIMBY, a nonprofit focused on the cause, through Stripe.

The pair bonded over having both lived in Zurich. After Mr. Sramek briefly worked as a consultant to Stripe in 2017, he embarked on his city idea, pitching roughly a dozen investors.

Patrick Collison was the first to bite, eventually amassing a stake smaller than 3 percent in the firm alongside his brother. He also connected Mr. Sramek to a circle of powerful investors, including the venture capitalist Michael Moritz, then at Sequoia Capital, who went on to solicit other investors.

The group decided to structure the company like an investment fund, with Mr. Sramek as the general partner and the investors as limited partners. They opted not to use debt, an otherwise common move in real estate investments, to give themselves more flexibility for a timeline that they thought could take several decades.

The venture firm Andreessen Horowitz also invested. In 2020, Marc Andreessen, the firm’s co-founder, wrote an impassioned 2020 blog post bemoaning skyrocketing housing prices in San Francisco and America’s inability to build cities.

The group discussed a variety of plans. It ultimately pursued the most ambitious one in hope that sheer size would increase the odds of success when trying to rezone farmland for residential use, the people familiar with the effort said.

Mr. Sramek chose Solano County for a number of reasons: Its land ownership was relatively concentrated, and it was still part of the Bay Area, said the people familiar with his plan. Solano is also the poorest county in the region, which would allow them to pitch voters on the promise of billions in investment and tens of thousands of new jobs.

This year, the group hired a city planner, Gabriel Metcalf, to lead a team of architects and designers. Mr. Metcalf is well known in Bay Area housing circles for the two decades he spent leading the San Francisco Bay Area Planning and Urban Research Association, a think tank that among other things advocates more and denser housing in the region. For years, he has been an outspoken critic of San Francisco planning. A spokesman for Flannery said Mr. Metcalf could not be reached for comment.

Under Mr. Metcalf, Flannery has expanded from a skeleton operation to roughly three dozen people, mostly in Northern California, working on areas like design, engineering and urban planning, according to the people familiar with the work.

As of August, the company had acquired more than 50,000 acres. County maps that show scattershot holdings for Flannery don’t paint a full picture, one person said, because the company has also struck arrangements with some landowners that would not have to be reported to the county.

“The vast majority of landowners in this area concluded that Flannery’s offers were simply too good to pass up and negotiated sales,” the company said in court documents.

Some offers have been summarily turned down. Mr. McConnell, the mayor of Vallejo, said the group had tried twice to persuade the board of the Solano County Water Agency, of which he is a member, to sell a large parcel that the agency had acquired for environmental remediation.

“They came a year and a half ago, and we said no, and then they came back a couple of months ago and doubled their offer,” Mr. McConnell said. Again the board refused.

After years of secrecy, the group has started on an apology tour. State Senator Bill Dodd, a Democrat who represents the area, said Mr. Sramek and one of the group’s political consultants, Andrew Acosta, met with him on Wednesday in Sacramento.

The pitch was scant on details: The pair told him that they would soon explain their vision, and that they felt they had the money and political sway to address the area’s concerns about the lack of water and clogged roads. He nevertheless remained skeptical.

“They did an act of contrition, and that was the start of the conversation,” Mr. Dodd said. “But they’re on a mission. They want to create another city in Solano County. I think they’re off to a very bad start.”

Shawn Hubler contributed reporting from Sacramento.