Boston is enlarging its staff of emergency medical technicians for the first time in seven years in an effort to improve response times.

Two classes of EMT recruits are in training. By the time they have all graduated in March, the Emergency Medical Services Department expects to increase its staff of full-time EMTs by 20, to 371.

The beefed-up staff will be paid for with a $2.5 million annual budget increase for the department. The money will also fund replacement of 10 aging ambulances in the city’s fleet of about 50.

“I’m pleased to welcome this new class of recruits and thank them for their service to our city,’’ Mayor Martin J. Walsh said in a statement. “Investing in our emergency medical services means investing in the health and well-being of all Boston residents.’’

In November, the Globe reported that response times to heart attacks, shootings, and other serious episodes had slowed in recent years, largely because of a little-understood surge in the number of calls for help.

City officials said in a statement that the increase in EMS funding and staffing “will help meet the heightened demand for emergency medical services and reduce the median response times for the city’s highest priority calls.’’

The staff expansion will allow the city to roll out either one or two additional ambulances, depending on the time of day.

City officials said it’s the first time EMS has ever run two classes of recruits at the same time.

The men and women in training are all state-certified EMTs. Prior to training, they went through a hiring process that included exams and an interview.

Boston EMT recruits go through a six-month training academy that begins with three months in a classroom setting followed by three months in the field, working in ambulances.

The training expands the EMTs’ clinical skills and prepares the recruits for “scenarios beyond the routine, including mass casualties, hazardous materials exposures, and active shooter incidents,’’ the city said.

Only those who successfully complete the training will graduate and be accepted by the department.

Matt Rocheleau can be reached at matthew.rocheleau@globe.com. Follow him on Twitter @mrochele