Since angles A and B are complementary, we know that:
\[ m∠A + m∠B = 90° \]
Given that \( m∠A = x° \) and \( m∠B = 2x° \), we can substitute these values into the equation:
\[ x + 2x = 90° \]
This simplifies to:
\[ 3x = 90° \]
Now, to find \( x \), we divide both sides by 3:
\[ x = \frac{90°}{3} = 30° \]
We have found \( m∠A \):
\[ m∠A = x = 30° \]
Next, we calculate \( m∠B \):
\[ m∠B = 2x = 2(30°) = 60° \]
Therefore, the measure of \( ∠B \) is
\[ \boxed{60°} \]