Similar sounding call signs and high density takeoff/landing operations resulted in communications confusion and a possible runway incursion at ORD.

Narrative

Our flight [12X1] was operating as ATC call sign [2X1A]. We followed our taxi instructions out to Runway 22L via B and D. Meanwhile; Company flight [12Y1]; operating in the new paint scheme; was cleared to Runway 22L via M. We; 2X1A; had been advised by Tower of our sequence for takeoff behind the ERJ flight in front of us and were told to be ready to go. Tower had been launching 2 to 3 flights between arrivals; we would be the 3rd and last before the next arrival.The ERJ in front of us was cleared for takeoff; and I heard Tower say; '2X1A line up and wait; be ready to go.' As I was turning on the lights for takeoff; I heard my First Officer respond to Tower and I started to taxi out onto the runway. As I taxied onto the runway I heard Tower say; '2 aircraft answered that call; it was only for Company 12Y1.' I immediately advised Tower; 'Company 2X1A took that call and now we're in the way.' Tower subsequently cleared us; [Company 2X1A]; for takeoff and no further discrepancies occurred.I firmly believe that the original takeoff clearance was for us (2X1A). Also; I do not know who the other aircraft that answered was; because Company 12Y1 (who was sitting off our right and pointed toward us) never turned on any lights; started moving; or made any indication that they thought the call was for them. Further; we had clearly been told our sequence before hand.The cause of this incident was the similarity of flight numbers 12X1 and 12Y1 with the non normal call sign thrown in; as well as the close physical proximity of those flights. The Tower Controller was doing an excellent job of launching aircraft that evening. He had a very long line on the ground; with inbound aircraft to the same runway. I don't know what other aircraft answered the takeoff clearance; but I'm sure this tripped the confusion. I still believe we were cleared for takeoff; but then he just read the next Company flight number accidentally when questioning the '2 responses' he had received to the takeoff clearance.