Walmart is betting on grocery-shopping robots to help it win the battle against Amazon and Kroger for customers' food spending.

The company has been quietly testing the machines, which are part of a system called Alphabot, in a small town in New Hampshire for months. Walmart is now preparing to roll out Alphabot, which can pick grocery orders up to 10 times faster than a human, to two more stores.

I traveled to the test site this week to get a behind-the-scenes look at Alphabot in action and see how the world's largest retailer is preparing for the future of online food shopping.