BENTONVILLE, Ark. — Wal­mart has made several changes in its management ranks in recent weeks.

The biggest of those changes has been the appointment of Walmart U.S. chief operating officer Bill Simon to president and chief executive officer of the domestic unit.

The 50-year-old Simon takes over from Eduardo Castro-Wright, who had headed the United States unit and will move to California to be with his wife, who is recovering from a heart transplant.

Castro-Wright, 55, will remain as Walmart’s vice chairman and will continue to oversee the company’s global e-commerce initiatives and global sourcing, two key areas for Walmart as it looks online and overseas for growth.

Simon’s elevation to chief executive officer comes at a time when Walmart is struggling with sluggish sales as its core customer continues to be pressured by high unemployment. Sales at U.S. Walmart stores open at least a year have fallen in each of the last four quarters.

Walmart has attributed the downturn in its business to both the economy and its own missteps. For instance, executives say that a move to pare the number of items carried in Walmart stores has cost the company sales, as consumers who could not find their favorite items began to shop elsewhere.

The company has been restoring some items and has announced price cuts on thousands of products in a bid to boost domestic traffic.

After being named chief executive, Simon said he would continue Walmart’s ongoing efforts to build traffic and ensure the company’s stores have the right product assortment.

Meanwhile, Walmart chief merchandising officer John Fleming has left the company.

Fleming, who had been with Walmart for a decade, was, in Simon’s words, “the architect of groundbreaking efforts to grow our e-commerce business, creative marketing programs to communicate with customers and inventive merchandising initiatives.”

Simon says executive vice president and general manager of grocery Jack Sinclair will now oversee company’s merchandising efforts in food and health and wellness, while executive vice president of planning, pricing and replenishment John Westling will oversee Walmart’s general merchandising and replenishment efforts.

“Our leadership team will focus on picking up our pace to meet our challenges,” Simon said in a letter to Walmart associates two days after he was named chief executive.