A solid rectangular block is formed by gluing together $N$ congruent 1-cm cubes face to face. When the block is viewed so that three of its faces are visible, exactly 231 of the 1-cm cubes cannot be seen. Find the smallest possible value of $N$.
1. Let the dimensions of the rectangular block be \( x, y, \) and \( z \). When viewed such that three of its faces are visible, exactly 231 of the 1-cm cubes cannot be seen. This means that the cubes that are not visible form a smaller rectangular block inside the original block with dimensions \( (x-1), (y-1), \) and \( (z-1) \).

2. The volume of the smaller block is given by:
   \[
   (x-1)(y-1)(z-1) = 231
   \]

3. We need to factorize 231 to find possible values for \( (x-1), (y-1), \) and \( (z-1) \). The prime factorization of 231 is:
   \[
   231 = 3 \cdot 7 \cdot 11
   \]

4. To minimize \( N = xyz \), we should choose \( x, y, \) and \( z \) such that \( (x-1), (y-1), \) and \( (z-1) \) are as close to each other as possible. The factors 3, 7, and 11 are already distinct and cannot be made equal, but we can arrange them to be as close as possible.

5. Assign the factors as follows:
   \[
   x-1 = 3, \quad y-1 = 7, \quad z-1 = 11
   \]
   Therefore:
   \[
   x = 3 + 1 = 4, \quad y = 7 + 1 = 8, \quad z = 11 + 1 = 12
   \]

6. Now, calculate the volume \( N \) of the original block:
   \[
   N = xyz = 4 \cdot 8 \cdot 12
   \]

7. Perform the multiplication:
   \[
   4 \cdot 8 = 32
   \]
   \[
   32 \cdot 12 = 384
   \]

Thus, the smallest possible value of \( N \) is \( \boxed{384} \).