Amazon on Wednesday announced its next-generation Kindle Fire HDX tablets, which will ship in 7" and 8.9" versions on October 18. As you might expect, these devices are very price-competitive with the Android market leaders and are quite a bit less expensive—and more impressive technically—than Apple's iPad. They also neatly upstage the recently announced Surface 2.

"It's been just two years since we introduced the first Kindle Fire, and the team is innovating at an unbelievable speed," Amazon Founder and CEO Jeff Bezos said in a prepared statement. "We've worked hard to pack this much hardware, innovation, and customer obsession into these prices."

I'm not so sure about the "unbelievable speed" statement—given the once-a-year release cycle that seems to be standard in this market—but the new devices certain offer impressive improvements over their predecessors.

The 7" Kindle Fire HDX starts at $229, the same price as the Google Nexus 7 and a full $100 less than an equivalent Apple iPad mini. For that price, you get a quad-core 2.2GHz Snapdragon processor with 2GB of RAM and a 1920 x 1200 "HDX" screen that offers 323 pixels per inch. It runs for 11 hours in mixed use, Amazon says, or 17 hours when reading. The Nexus 7 offers similar specs but a slower processor; the iPad mini is not technically competitive. Amazon will ship the 7" Kindle Fire HDX to customers starting October 18, 2013.

The 8.9" version starts at $379 and provides the same processor and RAM, but it's outfitted with a 2560 x 1600 "HDX" screen with 339 pixels per inch. Battery life is impressive, with Amazon claiming 12 hours in mixed use or 18 hours when reading. Google and Apple have yet to announce next-generation full-sized tablets, so comparing this device with the current offerings is a bit unfair. But the Nexus 10 starts at $399, whereas Apple's current-generation iPad with Retina Display starts at a still-eye-popping $499. The 8.9" Kindle Fire HDX will ship on November 7, 2013.

Both devices are thinner and lighter than previous-generation Kindle Fire HD tablets, and both are significantly lighter than comparable iPads.

Given Microsoft's consumer device aims, these new Kindle Fire HDX tablets represent a serious competitive threat. The recently announced Microsoft Surface 2 starts at $449 and features a slower processor, has a lower-resolution screen, and comes saddled with a far less impressive ecosystem of apps, services, and hardware accessories.

Related: "Microsoft Announces Second-Generation Surface Devices"

Amazon is also selling a new version of the previous-generation Kindle Fire HD, with a 7" 1280 x 800 screen, dual-core 1.5GHz processor, and 1GB of RAM, with prices starting at just $139. It will ship to customers on October 2, 2013.

Amazon's Kindle Fire lineup has always used a heavily modified version of the Android mobile OS, but this year it gets a name for the first time, Fire OS 3.0. According to Amazon, this OS, code-named Mojito, starts with Android and adds "cloud services, a content-first user interface, built-in media libraries, productivity apps, and low-level platform enhancements to integrate Amazon’s digital content and improve performance for Kindle Fire tablets."

Amazon notes that if an app runs on Android, it "can" run on Kindle Fire HDX. This doesn't mean it "will" run on these devices, however, since Amazon makes it difficult for users to add the Google Play Store to the devices.

On a related note, Amazon previously announced a new-generation Kindle Paperwhite eBook reader. This device starts at $119 and offers weeks of battery life. It will start shipping to customers on September 30.

All of the new Amazon devices—the Kindle Fire HDX tablets and the Kindle Paperwhite—are now available for preorder from Amazon.com.

Related: "Is the Kindle Fire Gaining Traction in the Enterprise?"