Without changing history, and it being a year-round dry landmass: nearly impossible, however...
I can think of two options.
OPTION 1: Cyanobacteria covering the entire landmass
These are bacteria which appear blue to cyan in color and can grow in a variety of different environments. The goal here is to have the landmass be covered by a bluish tarp which would ensure that you can't quite see it until you view the entire landmass at once, such as with a satellite. Ideally the type of bacteria covering would have three traits.
Firstly, the bacteria should be relatively toxic to humans. Having the bacteria be toxic ensures that any ships and people who reach the landmass are not able to tell anybody that it happens to exist. This would create a region which people avoid due to no know successful voyage through it but not direct knowledge of the continent.
Second, this bacteria should appear to be nearly indiscernible from ocean water but not exactly the same. The idea here would be that it seems that due to the similarity in color the water, from the surface or air where only a portion of the landmass is in view, is simply a sandbar, reef or debris present in the water. Once it is determined that the ENTIRE region is the same color and that color is not quite ocean water it would slowly become clear that this is in fact a very unique continent not a difficult to navigate portion of the ocean.
Finally, to constrain the cyanobacteria to the continent it should not be able to survive very well in ocean water. This will severely limit the potential for previous contact with the bacteria to occur which in turn will further enforce the idea that this portion of the ocean simply has weird colored water.
In summary: Cover the entire landmass in bacterial camouflage which is toxic to those who are unfortunate enough to reach the landmass
OPTION 2: Very, very specific geography and a series of barriers to navigation until the 20th century
In theory a landmass which has very specific geography could likely evade discovery as a continent until you have a clear picture of the entire Atlantic Ocean. Specifically if it was difficult to navigate during the age of sail, large enough for its extend to be indeterminate from a commercial plane and if the core landmass was surrounded by difficult to navigate waters.
There are a few stages to this:
First, basing this continent well inside (minimum 50 Km to deal with the horizon on ship which has, say a 100m mast) inside of a gyre should make it more difficult to reach in the age of sail in the first place until we have self propelled ships, since travel is slower inside of these regions it makes it more difficult to navigate to and possibly map the potential landmass.
Second, Make a large series of reefs, rocks, sandbars and barrier islands surrounding the continent which cannot be easily seen from inside the gyre, an area with established currents and good prevailing winds, such as the Sargasso sea. Also, you are not able to see ANY of the continent itself as of yet, and the closest island/sandbar/reef/rock needs to be at least 50 Km away as well. Ideally you would want to establish that this barrier region can only truly be navigated at all by VERY small watercraft, which can't reach the region unassisted. The reason for this is to make a barrier which is notoriously difficult to navigate which is inside of an area known for calm winds. The pairing of these two factors would nearly force sailors to avoid this area until the 19th century where you could then attempt to navigate a steamship inside of this region. Given the historic difficulty of navigating these waters major shipping would likely avoid the region. Since larger ocean worthy ships would not be able to pass this barrier region and there does not appear to be any landmasses in the region it becomes less likely that an expedition to this region would occur in the first place.
Third, the continent itself should not appear to be connected until you view the entire area at once, and width should be at ideally around 600 to 700 Km to ensure that the exact extent of the content can't be determined from a commercial plane. The geography of the continent comes into play here, if the continent is mostly marshland with very few to no obvious land-forms then even it would be very difficult to determine that it is in fact a continent-sized landmass from the surface. Given it is difficult to reach in the first place it is likely that nearly nobody would attempt to search for land within the region.
Optionally, like the first option, have most all the land area covered by various levels of special cyanobacteria or similar blue-green appearing organism. In this case, without viewing the entire area it would appear from above that the region just has strange colored water by anybody who gets close or only views a specific portion of the region by plane. The difference in the water color would become apparent only when the entire area is viewed as a whole by satellite.
In Summary: People would know that there was a large portion of the Atlantic which is difficult to navigate for one reason or another but can not determine that is a single connected landmass until viewing the area as a whole.
Both ideas would likely lead to indirect knowledge before the satellite photo but no confirmed discovery. In either option it would not seem likely that people would want to live in this region.
REFERENCE:
https://en.wikipedia.org/wiki/Horizon
https://en.wikipedia.org/wiki/Barrier_island
https://en.wikipedia.org/wiki/Horse_latitudes
https://en.wikipedia.org/wiki/Sargasso_Sea
https://en.wikipedia.org/wiki/ATS-3
https://en.wikipedia.org/wiki/Cyanobacteria