Well, the Boeing 777-300ER (that which you are calling the -300X), is a mere dozen years more advanced than the 747-400, so I don't know that I'd consider it an "inbread competitor" as much as a replacement. The 777-300's size restricts it very slightly - it can't carry quite as many pax. as the 747-400, but close enough that with its range it should be a powerful competitor. The 777 was originaly intended to be a between-the-767-and-747 size airplane, something for those who wanted great big but not behemoth. The 777-300ER will fill this market exactly as intended. With the 747-400ER now in production, the -300ER will, suffice to say, fill the market for airlines wanted just-smaller-than-747-400 size airplanes. In the mean time, Boeing is already giving itself a thorough headache over the 777-vs.-747 issue. The technological superiority of the 777 has convinced many airlines to opt for it over the 747. Why Virgin Atlantic doesn't operate 777s I simply do not understand. In any case, the Boeing is presently experiencing the 777 replacing the 747. A few very large airlines will still have an interest in the 747, but the 777 is tommorow's plane. Remember that the 747-400 was engineered in 1989. The main interest of the 747-400 right now is as a freighter, where it is vastly superior to most of the other available products. A 777-200F or -300F hasn't yet even been introduced by Boeing, and if it were it would be a while before they were put into service, and in that aspect the behemoth size of the 747 really does rule, so I think Boeing is being very intelligent about maintaining a 747-400F sales program whilst pitching the 777 to every airline market viable. So to answer your question, yes, the 777-300ER will "compete" with the 747-400, but no one is going to deny that, esp. not Boeing, they know what they've invented and they know where it is going.