Welcome to my fantasy, late-medieval world. For the sake of the question, we can divide it up in two major continents:
Continent A is considered the birthplace of my humans and features a solid presence of the other main race (winged humans, of which I talked in a previous question). The geopolitical landscape is divided in a human theocratic kingdom (ruling some of the mainland, the south and the underground cities), a federation of coastal merchant cities, and a somewhat loose union of winged-humans tribes controlling key-locations, mountain ranges, and the northern regions. Those factions seldom agree on something, and infact, have been mostly at war for the latest centuries.
Continent B has both humans and winged humans, some native of the place and some descendants of pioneers from A. Apart from some smaller kingdoms, B's much more uniform, the majority of the land is under the control of a feudal empire.
Now, all's fine and good.
A and B have trade routes via sea, the voyage being around two months long, a little less with good winds.
Suddenly, B invades A with airships. The airships are powered by a particular magic that flows in the air, but the focal point is that no one had been able to use that magic as a source of energy before.
A's kingdoms are dumbfounded. Nobody expected that. There were some signals that something was off (trade of some materials, mainly metals, somewhat diminished months before the invasion), but overall, nobody was expecting something like that.
If the kingdoms of A had spies at the court of B's emperor, they failed miserably.
So, How likely is that B has been able to develop a whole new mean of transport without most of A's noticing?
Additional infos and miscellanea:
- The airships are not extremely large, the largest being able to transport up to 50 men.
- Their main advantage is being able to fly (obv).
- They are not heavily armed. The airships mostly attack with crossbowmen manning the deck, occasionally pouring burning pitch over the enemy's structures, and balistas for the largest ones.
- Gunpowder doesn't exist in this world.
- The airships can fly as high as the winged-humans can; winged-humans man some of the airships, being valued most as navigators.
- B had never attempted to invade A before.
Clarification (as requested in the comments):
- The first step in developing the airship is coming up with the engine (the component able to harness magical energy). The task was researched by a small team of engineers (around five) under the leadership of a keen-on-innovation noble. This took a considerable amount of time (around twenty years) with a long list of failed attempts. Note that the engine didn't need to be tested on actual, fully functional airship.
- After the engine was built, the emperor took interest in the project, trying to bring everything under secrecy, informing the smallest number of people possible.
- For the first working prototype, I expect having at least two high-ranking officials, 5 engineers, and somewhat ten unskilled workers (maybe soldiers).
- For the mass-production of the fleet, I'm able to give you just an esteem, but I imagine we can expect five high-ranked officials to survey the whole operation, around 30 at least between engineers (including those from the previous team) and skilled artisans who need to have a larger view of the project.
- Everyone else would be on a need-to-know basis, e.g. the carpenters working at the ships hulls, or whoever tasked with getting those materials. For those, we're talking around 1300 for a major Arsenal (took the Arsenal of Venice at the peak of its splendor for comparison, from Wiki: Venice-Arsenal)