Amazon has announced that a large chunk of HBO's extensive and high-quality content library will be coming to its Prime Instant Video unlimited streaming service on May 21.

The company also announced that the HBO Go streaming app will be coming to Fire TV later in the year. The streaming app was notably absent from the streaming box at launch, despite its long-standing availability on Chromecast, Roku and Apple TV.

Until now, HBO has resisted licensing its content to any third party services, instead offering it exclusively to HBO subscribers through on-demand cable or the HBO Go streaming app.

The announcement intensifies the ongoing battle between Amazon Prime and Netflix. Both companies have put a premium on locking down exclusives and creating their own original content such as House of Cards (Netflix) and Alpha House (Amazon).

A shot across Netflix's bow

The new multi-year deal will bolster Amazon's content lineup with unlimited streaming of shows including The Sopranos, Six Feet Under, The Wire, Big Love, Deadwood, Eastbound & Down, Flight of the Concords, Enlightened, and Treme, along with mini-series such as Band of Brothers and John Adams. Many of HBO's comedy specials and documentaries will also be available.

Amazon subscribers won't get everything with this deal. HBO will continue to reserve the most recent three years of its programming for subscribers only, meaning no Game of Thrones or Girls yet. However, early seasons of Boardwalk Empire and True Blood will be available at launch.

To this point Netflix has been the favorite in terms of content lineup and been dominant in terms of subscriptions.

Adding HBO content is power move for Amazon, as viewers without a cable subscription are unable to buy HBO Go a la carte, which has historically put its content out of reach for cord cutters.

Access to this programming could be a key driver for people to invest in Amazon Prime subscriptions, which also includes free two-day shipping from the retail behemoth.