The brick-and-mortar mainstay has held the top spot in quarterly sales for more than a decade, according to analysts.

Amazon long ago passed Walmart in terms of market cap, but the e-commerce giant is finally poised to leapfrog its brick-and-mortar rival by another key metric: revenue.

It's no coincidence that these three retailers run massive deal days within weeks of one another, as with Amazon Prime Days and Target Circle Week. All three also now offer some of the fastest online shipping speeds available.

The clash of the retail titans has entered a new phase: for the first time, Amazon (AMZN) has surpassed Walmart (WMT) in annual revenue. The online retailer famously founded by Jeff Bezos in his garage during the 1990s has long led Walmart in market capitalization but hadn’t passed Walmart in earnings.