(Andrew Harrer/Bloomberg)

Amazon announced Thursday that members of its Prime program in select cities will now have access to free same-day delivery on certain items, a move that broadens Amazon’s subscription offering at a time when new players are poised to provide fresh competition.

In 14 metro areas, including Washington and Baltimore, Prime shoppers who place orders before noon will be able to get free same-day delivery on orders over $35 of items such as pet food, computer chargers and books.

Amazon expects that plenty of customers will continue to use the free two-day shipping offer that has long been the promise of the Prime program.  Some 20 million items are available for two-day shipping, compared to the 1 to 4 million that will be available for same-day delivery.  An even smaller assortment of items, “tens of thousands,” the company says, are available for one-hour delivery in certain markets.

“We’re constantly working to build alternatives to let customers choose what fits their needs and wants,” said Greg Greeley, the vice president of Amazon Prime Global.

Amazon has been experimenting with same-day delivery for several years in some cities; until now, it was available for $5.99 per order for Prime members.

Amazon’s latest addition to its Prime program comes as buzz is mounting about two new e-commerce membership offerings.  Wal-Mart has said it will begin testing this summer on a shipping membership program that would cost $50 a year and promises shoppers free three-day shopping on 1 million of its top-selling items.  Another site, Jet is set to debut later this year with a slightly different model:  It promises shoppers that for a $49 annual fee, shoppers will get access to the lowest prices on the Internet.

(Amazon’s chief executive, Jeffrey P. Bezos, owns The Washington Post.)

In addition to competition from retailers, Amazon is also starting to see other major tech firms make a bid for the e-commerce world by augmenting their online payment systems.  Google is adding a “Buy” button to its search results, which could help the firm bolster its advertising revenue, particularly on mobile devices. Google’s chief business officer Omid Kordestani said that launch was “imminent” at a major tech conference Wednesday.