Maximum vertical forces produced by flying animals can be difficult to identify unequivocally, but potentially indicate general limits to aerodynamic force and muscle power output. We used two methods (i.e. incremental addition of supplemental mass and asymptotic load lifting) to determine both the intraspecific allometry of and methodological differences in estimates of maximum flight performance for the bumblebee Bombus impatiens. We found that incremental mass addition underestimated maximum lifting capacity by approximately 18% relative to values obtained by asymptotically increasing the applied load during a lifting bout. In asymptotic loading, bumblebees lifted on average 53% of their body weight, and demonstrated a significantly negative allometry of maximum aerodynamic force production relative to thoracic muscle mass. Estimates of maximum body mass-specific mechanical power output increased intraspecifically with body mass to the 0.38–0.50 power, depending on values assumed for the profile drag coefficient. We also found a significant reduction in vertical force production when both hindwings were removed. Limits to load-lifting capacity ultimately co-occur with an upper bound on stroke amplitude (∼145 deg.). Although thoracic muscle mass showed positive allometry, overall load-lifting performance exhibited significant size-dependent degradation.