Not for the first time in his career, a problem of resources was preventing Masayoshi Son, the billionaire Japanese technology investor, from realising his ambitions.

His company SoftBank had recently stretched its balance sheet to splash $32bn on UK chip designer Arm. But the serial dealmaker, who believes in his ability to see into the future, wanted more.

So where would Mr Son find the big sums still needed to act on his latest vision of a world where humans, devices and the internet are more closely integrated?

The answer arrived on 13 planes. They carried a 500-strong state delegation from Saudi Arabia to Tokyo last month. Leading it was the hard-charging reformist Prince Mohammed bin Salman al-Saud, known by his initials MBS.

Luckily for Mr Son, the 31-year-old was a man in a hurry with his own programme — “Vision 2030”, targeting a radical modernisation of the Gulf country.

Some six weeks later, the two men would meet in Riyadh to launch plans for the largest private fund of its sort ever created — a $100bn partnership that will allow Mr Son to invest in the future of technology and for Saudi Arabia potentially to reap the benefits.

On paper now is a non-binding agreement for the creation of a SoftBank subsidiary based in London. Over the next five years, Saudi Arabia has committed up to $45bn, while SoftBank has promised at least $25bn. A further $30bn is being sought from “a few large global investors”.

The initial reaction to the SoftBank Vision Fund has been a mixture of awe and scepticism. Awe since the scale is equal to all the money raised by US venture capital industry in the past 30 months. Scepticism because questions abound as to whether the parties are truly committed to funding and seeing through the unprecedented project.

Even the insinuation of doubt, however, draws a rebuke from those closest to the plans. “What this isn’t is a couple of guys throwing money around,” says one of these people. Another adds: “This was a marriage of minds. The Saudis want to bring technology to their country and Masa [Mr Son] wants to be the largest technology player in the world.”

The unlikely marriage led from a courtship during the state visit to Japan in early September. Mr Son had been considering innovative financing arrangements for months, given the scale of his ambitions and his frustration with public market investors, who he feels take too long to understand his big “crazy ideas”. When SoftBank decided to purchase the UK’s Arm, it was slowed by the need to liquidate parts of its holdings to fund the transaction, which also pushed the company’s net debt to a staggering $105bn.

To solve the problem, Mr Son enlisted SoftBank executive Rajeev Misra, an ex-debt trader who in 2006 helped SoftBank structure a complicated takeover of Vodafone, Japan, to explore alternative pools of capital. Mr Misra, who will lead the new fund, asked two former colleagues from his time at Deutsche Bank — Nizar Al-Bassam and Dalinc Ariburnu — to field interest from potential partners.

Senior Saudi officials were among those contacted by Mr Al-Bassam and Mr Ariburnu. In short order, a series of meetings with SoftBank were arranged during the Saudi delegation’s visit.

Before seeing MBS, Mr Son honed his pitch with some of the royal court’s closest advisers: Yasir Alrumayyan, the secretary-general of the state Public Investment Fund (PIF); Khalid al-Falih, the influential energy minister and chairman of state oil company Saudi Aramco; and Majed Bin Abdullah Al Kassabi, the commerce and investment minister. Each pressed the SoftBank founder about the strategy and how it could fit in with their attempts to overhaul Saudi Arabia’s economy and reduce its reliance on oil.

Not only did Mr Son respond with examples of how SoftBank could help with the transformation of the country, but the technology mogul also boasted about his investment record and his ability to pick winners including China’s ecommerce leader Alibaba, mobile gaming company Supercell and Yahoo Japan.

After a courtesy visit with the Japanese emperor, meetings with the chairman of Hitachi and the heads of Japanese banks, it was time for MBS — an admirer of Japanese culture — to hear Mr Son’s presentation. Joined by his advisers, the deputy crown prince was hosted at the palatial Geihinkan state guesthouse in central Tokyo, which is designated as one of the country’s national treasures.

The meeting was a resounding success, captured in a photo of MBS with a smiling Mr Son. “The plans really took on a life of its own after Masa and the deputy crown prince met … Masa really made an impact on him,” one person involved says.

MBS left with a request for Mr Son to visit Saudi Arabia in October and spend time there better to understand the country. Saudi officials and advisers were dispatched to conduct due diligence confidentially on SoftBank and its portfolio companies, including the team behind its robotics division and robot Pepper.

A Japanese official said Mr Son found a kindred spirit in MBS, who projects an image of youthful purpose in a kingdom that has grown accustomed to a slow pace of consensual rule. “MBS also thinks that the speed of decision-making is very important,” said the official. He added that Mr Son’s contact book among tech entrepreneurs would dovetail well with the financial firepower that Saudi Arabia wants to deploy in the coming years, despite economic challenges at home because of low oil prices.

Yoshiki Hatanaka, a Middle East expert and adviser at International Development Centre of Japan, says: “In order for Saudi Arabia to attract investment in the future, it needed a flagship and eye-catching deal with someone who is a quick decision maker and internationally well-known. There are few in Japan other than SoftBank.”

Last week, Mr Son arrived in Saudi Arabia to make good on his promise of a multi-day visit. He toured oilfields in the country’s eastern province and visited Aramco, where he learnt about the state oil company’s engineering and robotics capabilities. Mr Son was particularly impressed with an Aramco robot that is able to clean solar cells, one person says, adding that a number of other partnerships beyond the fund are under way between SoftBank and the Saudis.

Time was also spent with the PIF, the Saudi sovereign wealth fund, and its leadership. Although the source of the Saudi funding has not been disclosed, the contribution is likely to consist of money from the PIF and later from the listing of Aramco, which is expected to be the largest initial public offering ever when it takes place in 2018 and raises tens of billions of dollars.

More controversially, Saudi Arabia will probably draw funds for the project from its liquid securities, fixed-income and currencies holdings, which are controlled by the Saudi Arabian Monetary Agency. PIF is taking over a large part of Sama’s $562bn reserves, a banker close to the Saudi government says, thereby giving the new wealth fund greater financial firepower to fund deals such as the SoftBank tech fund.

By the end of the visit, the two men racing to remake their respective worlds had consummated a deal and built a significant chemistry, although the specifics of their business relationship and the future direction of investments remain something of a mystery.

“Masa believes there are enough things out there to buy,” says someone who has worked closely with Mr Son.

“It won’t be classic leverage buyout or venture investing, it will be his own unique brand of identifying opportunities.”