It's known that both Canon and Nikon have been readying proper answers to Sony's and Fuji's mirrorless cameras, but details have been scarce and projected timelines aren't clear. Nikon recently clarified this, saying we can expect their mirrorless offering by spring of 2019.
In the interview, Nikon Executive Kimito Uemura directly confirms that "development is underway, and [they] expect to bring one to market by spring next year." It's likely that Canon is on some sort of similar timeline, as both companies have acknowledged the inroads Sony has made both in development and market success. The question on everyone's minds regarding both Nikon's and Canon's individual offerings is which lens mount each will have. The advantage both companies have over Sony and Fujifilm is a strong customer base heavily invested in their respective lens ecosystems, but the introduction of a new mount would at least partially negate this advantage. On the other hand, the physics of the flange distance presents difficulties in this regard, and both companies have had decisions to make on how to handle the issue. Rumor has it Nikon will be introducing the Z mount, though this has not been confirmed. Either way, 2019 should be a very interesting time for the market and consumers.