Royal Dutch Shell PLC’s $70 billion deal to buy BG Group PLC marks the most aggressive step yet in the competition to be the world’s dominant supplier of liquefied natural gas—a fuel with a fast-growing and increasingly global market.

The deal, announced Wednesday, would vault Shell far ahead of rivals like Exxon Mobil Corp and Chevron Corp. in the race to build market share for LNG, a chilled form of natural gas used for electricity generation and home heating.

The tie-up is a bet that countries like China, India and others in the developing world will move toward cleaner burning fuels like natural gas instead of coal amid growing pressure to curb emissions. And it is a gamble that Asian markets will come to rely on U.S. exports of the product, when the first shipments leave the country—expected sometime in late 2015 or early next year.

Exxon Mobil estimates that the global trade in liquefied natural gas will more than triple through 2040, to nearly 100 billion cubic feet a day—roughly 40% higher than current U.S. gas output. The company projects that countries throughout Asia and the Pacific will import half of the gas they consume by 2040, with LNG making up 80% of imports.

Building up its LNG business would also give Shell some insulation from volatile oil prices, which have plummeted since last summer.

“LNG is a very important component of this,” Shell CEO Ben van Beurden said on a conference call Wednesday, adding that BG was at the top of a list of companies Shell was considering buying.

In BG, Shell found a partner in a unique position to take advantage of growing demand for natural gas. BG has the rights to export the first sizable volumes of American LNG to foreign buyers through a long-term agreement with Cheniere Energy Inc. from the proposed Sabine Pass LNG terminal in Cameron Parish, La. Construction of that site is in the final stages, and the terminal is expected to begin exporting gas later this year. When Sabine Pass opens, it will be the first major U.S. LNG export facility.

Analysts said Shell’s deal for BG could provoke a reaction from Exxon, which has built a massive LNG production and trading arm and has bought back plenty of shares that could be used for an acquisition.

“The race is on,” said Dennis Cassidy, the head of oil and gas at energy consulting firm Alix Partners.

Shell said that, with BG’s assets, it would have roughly twice the LNG production volumes of Exxon by 2018.

Exxon declined to comment. A spokesperson referred to an interview Chief Executive Rex Tillerson gave to CNBC last month.

“We really are looking for opportunities where a company has great assets,” Mr. Tillerson said. “We don’t feel compelled to have to do anything. We already hold the industry’s largest resource base of anyone in the world.”

Shell agreed to buy BG at a 50% premium to its Tuesday share-price close. BG shareholders will get a 19% stake in the new company after the deal closes, likely in early 2016. To make it palatable to investors, Mr. van Beurden said the company would increase its dividend to $1.88 a share and launch a share buyback program of $25 million in 2017.

Still, Shell’s shares closed 5.3% lower in London, while BG stock ended 27% higher. Some analysts and investors were skeptical that Shell, which sat out megamerger waves in the past and has focused on exploration growth instead, could manage the acquisition.

“Shell’s track record of executing on acquisitions has not exactly been stellar over the past decade,” said Michael Hulme, commodities fund manager at Carmignac Gestion. “To assume that Shell can pay a 50% premium for BG, and extract significant synergies, deliver value for shareholders, and maintain a dividend on an expanded shareholder base would require a more-than-healthy degree of optimism.”

There are other potential risks. BG’s two most-prized assets face serious challenges. Its Queensland Curtis LNG project in Australia took a $6.8 billion write-down in the fourth quarter, and its Santos Basin Brazilian oil fields have as a partner Petróleo Brasileiro SA, the state-run company in the grip of a bribery scandal.

The deal is also notable for what it doesn’t include: a big position in North America with access to shale, the dense rock rich in oil and gas that has revived American energy production. Shell largely missed the shale boom.

But Shell executives said the combined group will have two strategic growth businesses—deep water oil and integrated gas—that could potentially each generate $15 billion to $20 billion of cash flow from operations a year.

The planned acquisition represents another step in Shell’s transformation from a company that mostly extracts crude oil from the ground to one that produces more gas than it does oil. The roots of the change can be traced back a decade, when Shell was regrouping from a financial scandal that saw its CEO leave and regulators fine the company for overstating its reserves.

In the aftermath of the meltdown, Shell invested heavily in natural gas and now says that integrated gas projects, in which Shell controls the extraction, shipping and end sale of natural gas, give the opportunity for long, steady profits.

And in internal planning discussions close to 10 years ago, executives decided to focus on gas in anticipation of government restrictions on carbon emissions, say people involved in those talks. Over the past few months, Mr. van Beurden has repeatedly called on governments to institute carbon limits, a development that would theoretically hasten a transition to gas from coal in places like India.

Shell eyed BG multiple times over the years, say those people and bankers, and Shell’s strategy team kept a file on BG that it reviewed periodically. The companies discussed a merger about five years ago that could have put BG’s then-CEO in charge of the combined company, these people said, but couldn’t reach an agreement.

The deal was first broached in a March 15 meeting between Mr. van Beurden and BG Chairman Andrew Gould, almost five weeks after Helge Lund had taken up the post as BG’s new CEO.

“It was very simple,” Mr. van Beurden said. “I called Andrew up and we had a very good and constructive discussion about the idea and it very quickly seemed to make sense to both of us.”

Write to Justin Scheck at justin.scheck@wsj.com, Selina Williams at selina.williams@wsj.com and Daniel Gilbert at daniel.gilbert@wsj.com

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