When MD11 sales were slow in the early 1990's MDC looked at several new variants including the MD12. MD12X, MDXX. Many people are i'm sure aware of the proposed MD12 which looked very similar to the Airbus A380 currently being built today. However, the MD12 started out as a tri-jet. The MD12X was also a tri-jet as were the MD-XX, MD-XX LR and MD-XX Stretch-all of which looked very similar to the MD11 and featured commonality in flight deck etc but featured a huge 213 ft wingspan, range of up to 8320 nm (LR) and a fusealge 31 ft longer than the current MD11. None of the proposed variants and indeed the new MD12 got off the ground, largely due to the huge debts that MDC were running. The MD-XX Stretch and MD-XX LR were unveiled at the Farnborough air show in 1996 but again the project never got the go ahead and shortly before the end of 1996 the project was cancelled.
Whilst the MD12 looks very similar to todays A380, MDC and Airbus did discuss the idea of a joint venture back in the mid 1980's but nothing came of it so MDC went it alone. Large parts of the MD12 were to be built in Taiwan in co-opertion with Tawain Aerospace as it was hoped and anticipated that Asian carriers would order the new aircraft. The deal with TAC fell through and the project cancelled in part due the the huge debts at MDC. Airbus would later launch their own double decker A380 which looks similar to the MD12. I haven't looked through any specs to see how simialr they could really be but it was MDC who led the way, albeit too far ahead of there time.