By Sebastian Herrera

At Amazon.com Inc.'s annual shareholder meeting in May, Chief Executive Jeff Bezos offered a familiar response when asked about scrutiny surrounding the company.

"We want people to know the truth about Amazon and how we use our scale for good," he said before outlining company initiatives on climate, job creation and small business.

That often-repeated message, and Mr. Bezos's tendency in public appearances and interviews to stick to folksy talking points about Amazon's rise, are set to be tested as never before when he makes his first-ever appearance before Congress on Wednesday.

In recent years, the world's richest man has granted few extended interviews to journalists and has seldom faced the kind of adversarial questions he is likely to get from lawmakers on the House Antitrust Subcommittee. He is set to testify via videoconference alongside CEOs from Apple Inc., Google parent Alphabet Inc. and Facebook Inc.

Mr. Bezos, whose net worth has soared to almost $180 billion, has ruled Amazon unchallenged since founding the company more than a quarter-century ago. He has a reputation for being unyielding when holding a strong belief or challenging what he considers to be poorly considered ideas. But he is also known for staying on message. Former Amazon executives say they expect Mr. Bezos to prepare carefully and to maintain discipline in sharing several messages that have come up frequently in the company's responses to scrutiny and criticism.

One is that while Amazon accounts for a large volume of e-commerce sales, its overall size in U.S. retail is much smaller. Another is its "customer obsession," one of 14 principles long touted by Mr. Bezos and his team as an explanation for the company's competitive behavior.

The crown jewel of his testimony "is very likely to be the consumer," said Guru Hariharan, who years ago helped build some of Amazon's seller services and now runs CommerceIQ, which works with brands selling on Amazon. "He can argue that you don't want to enforce too many rules on an open market because you will end up hurting the consumer."

A spokesman for Mr. Bezos declined to comment for this article.

Critics have questioned whether some of Amazon's practices put its own interests ahead of that customer-first principle, and lawmakers have been scrutinizing the company's competitive behavior as part of a broader examination of large technology companies including Amazon since last year. Momentum grew for Mr. Bezos to testify after a Wall Street Journal article in April detailed the use of third-party seller data by company employees to help create competing Amazon-branded products.

Amazon, which launched an internal investigation because of the Journal story, has said it prohibits employees from using nonpublic, seller-specific data when determining which products to launch.

On July 21, the company released a report written partially by its consumer chief, Jeff Wilke, that touts its investment into small businesses, saying it is spending more than $30 billion to support small and medium-size sellers on its site. The report also says Amazon works with more than 2 million independent sellers, authors, content creators, developers and other businesses in the U.S.

Rep. David Cicilline (D., R.I.), chair of the House panel, said lawmakers over the past year have "heard some very disturbing testimony about the experiences of innovators and small businesses with Amazon." Asked whether he was satisfied with how Amazon has answered questions about its use of third-party seller data, Mr. Cicilline said simply: "No."

"They are tremendous gatekeepers to a huge part of the economy," Mr. Cicilline said in an interview. "We want to look at in a very serious way the practices of Amazon."

Mr. Bezos's primary advisers for the hearing include top Amazon lawyer David Zapolsky and Jay Carney, Amazon's public relations and policy chief and a former White House press secretary to President Barack Obama.

Mr. Bezos's interview appearances are typically tied to company announcements or events, where interviewers avoid hot-button topics. One of his most recent interviews, for example, was held in conjunction with his visit to India in January, where he fielded friendly questions on stage from Amit Agarwal, head of Amazon's India operations. The interview broached favorite Bezos topics: Amazon's origin story and the company's pledge to eliminate its carbon footprint. His last lengthy interview with a journalist, a 2018 sit-down with Wired's Steven Levy, focused largely on his space company, Blue Origin.

Mr. Bezos often lets his subordinates defend Amazon. That includes executives such as Dave Clark, senior vice president of world-wide operations, and Mr. Carney, both of whom tweet out defenses of Amazon and appear on television interviews.

Mr. Bezos typically relies on his senior public relations staff, close executives and his legal team when preparing for important public appearances, former senior staff members said. Like many other executives, Mr. Bezos typically rehearses with his senior team members on the kinds of questions he could face. With the hearing being held virtually, Mr. Bezos will be able to confront questions from a much more comfortable setting than typical for congressional hearings, they said.

"He is someone who is very comfortable in his skin and knows what he believes and knows how he has built his company over time on very clear ideas and principles," said Scott Stanzel, a former director of communications at Amazon.

Amazon, whose share price has risen by more than 50% this year and has a market value of about $1.5 trillion, has frequently disputed the notion that the company's business practices drown out competitors. As the company has grown into the nation's second-largest private employer and attracted more strident criticism, it has put much more emphasis on telling its own story. It began a blog in 2017 that focuses on telling stories about its company efforts and staff. The company also has been directing more of its advertising efforts recently to highlight its relationship with employees.

Once known as a private family man, Mr. Bezos in more recent years has thrust himself into the limelight, although he has continued to avoid extended press interviews. He uses his Instagram account to communicate personal moments and messages.

The account includes content about Amazon's efforts to protect its workers and its pledge to be a leader in climate change prevention, appearing alongside photos of him with world leaders and music stars. According to former advisers, Mr. Bezos also crafts his image through promotional videos, his senior executive team and public relations representatives.

Major labor unions which have had conflict with Amazon, such as the International Brotherhood of Teamsters, have been lobbying committee members to get their issues heard at the hearing.

Amazon "has tremendous power to impact the supply of products in numerous categories at the expense of workers, sellers and consumers," said Michael Zucker, director of Change to Win, a federation of labor unions that has led efforts to pressure Amazon. "This power has been heightened during the pandemic, and it's a preview of the future that the online and general retail sectors are going to face if there is no law and regulation applied vigorously on Amazon."

Ryan Tracy

contributed to this article.

Write to Sebastian Herrera at Sebastian.Herrera@wsj.com