ABCNews.com

Walmart is rolling out free next-day delivery on its most popular items, increasing the stakes in the retail shipping wars with Amazon. The nation's largest retailer said Tuesday it's been building a network of more efficient e-commerce distribution centers to make that happen. The next-day service will cover 220,000 popular items from diapers and non-perishable food items to toys and electronics. That's nearly double the number of items it carries in its stores. Next-day delivery, which will require a minimum order of $35, will be available in Phoenix, Arizona, and Las Vegas on Tuesday. In coming days, it will expand to southern California. The discounter plans to roll out the service to 75% of the U.S. population by year-end. It will also be adding hundreds of thousands more products as the program expands. The announcement comes just two weeks after online behemoth Amazon said it's upgrading its free shipping option to Prime members who pay $119 a year to one-day delivery from two-day delivery.