By 2015, some 48 billion mobile apps will have been downloaded, according to researcher In-Stat.
The number will be bolstered by Apple's App Store, Google's Android Market and a number of smaller online stores, it said.
In-Stat said the rate of downloads has increased as the market for apps has become flooded. Apple's three-year-old App Store, for instance, now offers more than 425,000 apps while Android Market offers more than 200,000.
In-Stat did not specify its projected annual rate of increase in mobile app downloads, or offer historical figures.
However Juniper Research said there were only 2.6 billion smartphone app downloads in 2009. And Juniper predicted a year ago that there would be 25 billion smartphone app downloads by 2015.
Apple's App Store is a big reason for the explosive growth of app downloads. Apple said in July that more than 15 billion App Store apps had been downloaded to 200 million iPhones, iPads and iPod touches over the past three years.
By comparison, Google recently claimed 4.5 billion app downloads from its Android Market.
Philip Schiller, head of Apple product marketing, has said that Apple alone has paid out some $2.5 billion to app developers over the last three years.
The developers receive 70% of the purchase price of their apps sold on the Apple site. Apple's revenue from the site was $1.1 billion through the period, meaning that the App Store has generated some $3.6 billion in overall revenue through mid-2011.
In-Stat forecast that overall app download revenues will surpass $29 billion in 2015.
"Largely spurred by the launch of the iPhone, mobile applications have been a strong growth market over the past several years," said Amy Cravens, an In-Stat analyst. "The number of apps available has exploded, which has sparked an increasing rate of downloads."
She said the challenge for developers and app stores will be to compete across multiple mobile platforms.
In-Stat added that the explosive growth of app downloads will keep pressure on them to keep prices low, or even to offer free products. Users now pay an average of $2.50 for a mobile app, the research firm reported.
In-Stat said the spread of smartphones is also boosting app downloads. Smartphones made up one-fourth of all cellular phone shipments in 2010. That number is projected to jump to 45% in 2015, the company said.
Nearly half of all app downloads are now done over Wi-Fi, In-Stat said.