Normally, once u have been rejected US visa, u need to varify this info when you are filling the new form here in this embassy, or sometimes they put this remark on your passport, so they know under what section, they have rejected ur visa last time. But its better, u let them know that previous visa has been rejected, or else, if they find that u are giving false info,might blacklist your name...
Getting visa from qatar is not that difficult if you have company letter with you.