Wal-Mart Stores Inc. said it launched a free 30-day trial of ShippingPass, its two-day shipping program to all shoppers in the United States, as the world's largest retailer looks to take on Amazon.com Inc.'s Prime subscription service.

Wal-Mart also said existing ShippingPass holders will get a month free, as the company builds on initiatives it announced earlier this month.

walmart Shoppers cart their purchases from a Walmart store in Mexico City, April 24, 2012. Photo: Reuters/Edgard Garrido

The company had announced a slew of programs, including online grocery pickup and a partnership with ride-hailing app Uber to deliver groceries as it looks to boost online sales.

The ShippingPass, priced at $49/year, looks to compete with Amazon.com's Prime subscription, which costs $99/year.