Google’s Android operating system moved into second position in the U.S. smartphone market in this year’s first quarter, overtaking Apple’s mobile OS and trailing RIM’s BlackBerry, according to the latest wireless market research from the NPD Group.

Android was running on 28% of U.S. smartphone handsets as of the end of Q1, Apple’s OS on 21% and RIM’s on 36%, according to NPD’s report, which is based on last quarter’s unit sales to consumers.

Promotions and partnerships with handset manufacturers and mobile network operators were behind Android’s gains. Verizon Wireless’s promotional sales of the Droid, Droid Eris and Blackberry Curve helped the network provider keep up with AT&T, which has exclusive rights from Apple to sell the iPhone in the U.S. Sales of Verizon Wireless smartphone sales made up 30% of the U.S. total in Q1, AT&T 32%, T-Mobile 17% and Sprint 15%, according to NPD’s Mobile Phone Track.

“As in the past, carrier distribution and promotion have played a crucial role in determining smartphone market share. In order to compete with the iPhone, Verizon Wireless has expanded its buy-one-get-one offer beyond RIM devices to now include all of their smartphones,” according to NPD executive director of industry analysis Ross Rubin.

In addition, mobile phone prices rose slightly during the quarter while unit sales dropped. The average selling price in Q1 was $88, a 5% rise year-to-year, while smartphone unit prices rose 3% to $151, according to NPD.