Google is growing concerned about Samsung’s dominance of the Android smartphone market, according to the Wall Street Journal.  Last year, the Korean electronics giant won nearly 40 (39.6%) percent of the Android device market — far more of other Google partners.

Samsung brings in millions to billions of dollars to Google’s Android coffers every year, so it wields the power to get what it wants.  And some Google executives say they worry that if Samsung becomes more powerful, it might follow in the footsteps of Amazon by developing its own version of Android and its own walled garden of mobile content, which would hurt Google’s lucrative mobile ad business.

Google’s Android chief Andy Rubin last year reportedly praised Samsung’s success as one of Google’s manufacturing partners, but he also warned that Google would be endangered if Samsung pull too far ahead of competitors.  According to the Wall Street Journal, Google’s buyout of Motorola Mobility was an insurance measure (or in Rubin’s words, a “hedge”) that could prevent Samsung from gaining too much power in the Android arena.

So far, the Motorola devices we’ve seen under Google’s ownership aren’t exactly Jesus-phone caliber, although Google is reportedly in its final stages of work on a so-called “X Phone” that might finally give Samsung some strong competition. At the least, we expect that the “X-Phone” (and perhaps an “X-Tablet”?) will top Google’s Nexus phones and tablets which, although well-rated, are far from perfect.

With Samsung set to unveil its much-anticipated Galaxy S 4 Android phone on March 14, it’s a safe bet that Sammy’s lead will only increase over the next several months, making Google execs sweat even more.  Oh, “X-Phone,” where are you now?