Will let developers pay to promote their software As growth slows for Google Inc., the search giant is pulling an important new sales lever. In a blog post on Thursday, the company said it will let software developers pay to promote their apps in the Play store for the first time. It described the program as a “pilot” involving a limited number of users. The Play store, Google’s GOOG, +0.53% GOOGL, +0.60% version of Apple Inc.’sAAPL, -1.50% App Store, is home to more than one million apps, plus games, movies and more for phones running Google’s Android operating system. Google built a $50 billion-plus business around advertisers promoting products and services to users based on their search queries. Until now, it hasn’t allowed app developers and others to promote their wares in the Play store. Instead, it has relied on human curators and algorithms to choose the apps and other content featured on the store’s virtual shelves. Apple doesn’t have ads in its App Store. “There is a very large and established market for mobile-app marketing that Google has not had an attractive ad product for — up until now,” said Citigroup analyst Mark May. He estimates that Google could generate up to $3 billion in new, “high-margin” revenue by 2016 from Play Store ads, though he cautions that will depend on a number of factors including how fast Google scales up the new ad program. An expanded version of this report appears on WSJ.com