(REUTERS/Joshua Lott)

Wal-Mart is the biggest retailer in the world with $482 billion in annual sales, making it five times as large as its closest rival.

But there's one area where the retailer is perpetually beat: ecommerce.

Wal-Mart's online sales are a tiny fraction of the size of Amazon's, even though Wal-Mart is five times bigger — and a lot older — than it's ecommerce rival.

Wal-Mart's online sales were $12.2 billion in 2014. By comparison, Amazon's were $89 billion.

The world's largest retailer is tired of losing to Amazon when it comes to web sales, so it has been pouring billions of dollars into its online operations in recent years.

This year, Wal-Mart plans to invest between $1.2 billion and $1.5 billion in its ecommerce business, Wal-Mart Chief Financial Officer Charles Holley told analysts in February. The company will spend even more next year, he said.

Wal-Mart has been using the money to make its website more user-friendly, open additional ecommerce fulfillment centers around the US, and expand the number of products available for purchase on its website.

Wal-Mart also recently began authorizing store managers to match prices found on Amazon and other websites to better compete with its rivals.

And Amazon's growth continues to outpace Wal-Mart's, meaning the retailer is a long way from catching up.

(BI Intelligence)

Now the company is using a more direct tactic by attacking Amazon head-on and trying to erode its loyal customer base.

After Amazon announced that it would have a Black Friday-style sale for Prime members last week, Wal-Mart stole its fire by offering its own shopping event — starting on the same day — with thousands of online-exclusive discounts.

In announcing its sale, Wal-Mart chided Amazon for making "Prime Day" available only to Prime members, who pay $99 a year for free two-day shipping and other perks.

"We've heard some retailers are charging $100 to get access to a sale," Walmart.com CEO Fernando Madeira wrote in a blog post. "But the idea of asking customers to pay extra in order to save money just doesn’t add up for us. We're standing up for our customers and everyone else who sees no rhyme or reason for paying a premium to save."

The attack got a rise out of Amazon, which responded by accusing Wal-Mart of charging customers higher prices in stores than they do online.

(Thomson Reuters) Residents shop at Walmart as the store prepares for Black Friday in Los Angeles, California.

Wal-Mart has historically been very aggressive about going after its competitors.

According to Wal-Mart founder Sam Walton's book, "Made in America," the company was built almost entirely off of other retailers' good ideas.