Amazon is expanding its delivery network to offer same-day and next-day delivery to what it says are tens of millions of customers in more than 4,000 smaller communities across the U.S. by the end of the year.

Amazon is adding more than 4,000 "smaller" communities to its same-day and next-day delivery coverage — a move that challenges Walmart.

Millions of people in busy cities have access to same-day delivery services, but options are more limited in rural areas. Amazon wants to change that by extending same-day and next-day deliveries to more than 4,