Trying to create a new column using multiple if else statements in R

I would like to create a new column that classifies a person's BMI into certain categories.

I am not sure where I am going wrong:

brfss2013 <- brfss2013 %>%
  mutate(bmi_class = if (X_bmi5 < 18.5) {
    X_bmi5 == 'underweight'}
    else if (X_bmi5 in range(18.5,24.9)){
      X_bmi5 =='normal'} 
    else if (X_bmi5 in range(25,29.9)) {
      X_bmi5 =='overweight'}
    else if (X_bmi5 in range(30,34.9)){
      X_bmi5 =='class 1 obesity'}
    else if (X_bmi5 in range(35,39.9)){
      X_bmi5 =='class 2 obesity'
    else if (X_bmi5 > 39.9){
      X_bmi5 == 'class 3 obesity')}
      else 'NA')