How much prewar thought lay behind MAC ships

Were merchant aircraft carriers planned before the war or improvised after it began?

They were wartime expedients, but prewar professional discussion and carrier exercises had already developed much of the underlying idea.

The evidence does not establish that the Navy possessed finished prewar MAC-ship plans. Naval constructors and civilian architects had nevertheless discussed aircraft-carrying merchant hulls and related escort problems before the emergency. Exercises with Argus also gave the service experience of covering convoys beyond land-based air range. Wartime designers could therefore assemble familiar components quickly even though the final ships remained an expedient.