Amazon is increasing the core Prime benefit of free two-day shipping to one-day shipping. (GeekWire Photo / Kurt Schlosser)

Saying that every day is “Day 1” at Amazon might still be a popular mantra at the company, but today the tech giant is making sure its Prime customers are aware that it’s also “one day,” as in free one-day shipping for members of its subscription program.

For the first time the company is sharing just how many items are now available to be delivered in such rapid fashion. In the process, the company is effectively throwing down the gauntlet for Walmart, one of its biggest retail rivals.

Amazon said Monday that free one-day shipping is available to Prime members on more than 10 million products, with no minimum purchase amount.

That number comes just over a month after Amazon first revealed, during a quarterly earnings call, that its core Prime benefit of free two-day shipping would be getting an upgrade.

A few weeks later, Walmart’s U.S. e-commerce CEO Marc Lore announced that the retailer was launching free “NextDay” delivery in Phoenix, Las Vegas and Southern California, with plans to reach 75 percent of the U.S. population this year. Although it doesn’t require a membership, as Amazon does with Prime next-day delivery, Walmart’s next-day delivery program requires a minimum purchase of $35 of eligible products. Walmart said at the time that 220,000 items were available for next-day delivery.

Amazon’s announcement Monday makes clear that the selection in its Prime Free Next Day program is well ahead of Walmart’s selection thus far.

Lore, who worked at Amazon after selling Diapers.com parent Quidsi to the tech giant, sold his next startup, Jet.com, to Walmart. Although Amazon was first to go public with its next-day delivery plans, Lore was careful in his earlier announcement to describe Walmart NextDay as “a new offering we’ve been working on for a while now,” so as not to look like a direct response to Amazon.

Amazon first turned two-day delivery from what it called “an occasional indulgence” into an everyday experience back in 2005, offering one million products under that distinction at the time. Fourteen years later, the one-day effort is 10 times bigger.

In Q4, the company announced that it expanded same-day and one-day delivery to more than 10,000 cities and towns, and in April, said it would significantly expand its one-day selection.

And Amazon simply credits the fact that it’s been building out its network for over 20 years, ticking off a list of what brings goods closer to customers across the U.S.: 110 fulfillment centers, 40 package sortation centers, 100 delivery stations, and 20 air gateways.

“We have been offering faster than two-day shipping for Prime members for years — one day, same day, even down to one- to two-hour delivery from Prime Now,” Amazon chief financial officer Brian Olsavsky said in April, adding that the company had a head start and was moving quickly.

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