Amazon is expanding its 30-minute delivery service to dozens of U.S. cities, after first piloting the option in December.

The Amazon Now service will be available to "millions" of customers by the end of the year, according to Amazon. The tech giant first tested Amazon Now in Seattle and Philadelphia. Next, it will be widely available in Atlanta, Dallas-Fort Worth and Austin.

Amazon said the option will also be available in Denver, Houston, Minneapolis, Oklahoma City and Orlando, though it may be rolled out in parts of the cities first. The company will add additional areas to reach "tens of millions of customers" by the end of the year, the company said. Customers can check availability for the service at www.amazon.com/now.

Amazon's new 30-minute delivery builds on the retailer's existing 1-hour and 3-hour delivery and same-day delivery services.

Where available, Amazon Now will offer 30-minute delivery. Prime members pay $3.99 per order and customers without prime will pay $13.99 per order. For orders below $15, a small-order fee of $1.99 for Prime members and $3.99 for non-Prime members will be charged.