Hulu will soon join Amazon Prime and Netflix in allowing you to download TV episodes to your mobile devices and watch them while offline.

The new feature is a boon for frequent air travelers, public transit riders, and others who spend time separated from speedy Internet connections. Hulu made the announcements at the upfronts, events in which TV broadcasters pitch upcoming programming chiefly to advertisers but also to the press.

As noted, Netflix and Amazon also offer this feature. However, Hulu will be the first to offer it in an ad-supported tier. Those who pay for Hulu's $11.99/mo ad-free subscription will have the same ad-free experience as they would on Netflix and Amazon. But subscribers to the $7.99/mo ad-supported tier will actually download the ads right alongside the primary content, and they'll play during the episodes the same way they would on the Web or on set-top streaming boxes.

Hulu claims to be the first company in the TV business to offer downloadable ads.

Advertising is key for Hulu because—unlike Amazon and Netflix, which create their own streaming-specific content or license broadcast content long after it has aired—Hulu streams the same broadcast TV shows you see on networks like NBC or ABC—usually the day after air. For those shows, broadcast TV ads still account for the bulk of the revenue, and streaming ads are sold alongside the much more valuable broadcast ad buys in many cases.

Hulu is partially owned by those broadcast networks. The Walt Disney company, which owns TV network ABC, owns 30 percent of the company. 21st Century Fox owns another 30 percent. Comcast (which owns NBC Universal, which in turn owns the NBC broadcast network) owns another 30. The final 10 percent belongs to Time Warner via the Turner Broadcasting System, which also owns several popular cable TV channels like Cartoon Network, CNN, TBS, and TNT. The only major network that is not invested in Hulu is CBS, which has its own streaming service called CBS All Access. CBS also owns The CW, another major broadcast network.

In a way, Hulu was an attempt by those companies to jump into streaming without shaking up the business model that has driven their success since the 1930s and 1940s. So to offer this feature, Hulu had to come up with an option that accounted for ads.

Hulu hasn't said which shows and movies will be available for offline viewing. Netflix and Amazon currently only offer limited portions of their libraries. As with those two, you can probably expect all of Hulu's original series to be downloadable. The fact that an ad-supported option is available could be an advantage in onboarding more licensed shows to the feature.

In addition to offline viewing, Hulu announced that it has surpassed 20 million subscribers. The company has also, as is the main focus at the upfronts, unveiled its lineup of upcoming original series. The Handmaid's Tale will get a new season, and new series titled Four Weddings and a Funeral, Castle Rock (a project from Stephen King and Star Wars: The Force Awakens Director J.J. Abrams), and Catch-22 from George Clooney will hit the service.

Hulu said the offline-viewing feature will launch during the 2018-2019 upfront season.